Philip D. Gregory - 11 Nov 2021 Form 4 Insider Report for bluebird bio, Inc. (BLUE)

Footnote F1

Represents restricted stock units awarded to the reporting person prior to the separation (the "Separation") of 2seventy bio, Inc. ("2seventy") from bluebird bio, Inc. ("bluebird"). In connection with the Separation, the restricted stock units underlying this award were adjusted pursuant to the Employee Matters Agreement by and between 2seventy and bluebird (the "EMA") such that the reporting person received restricted stock units of 2seventy to preserve the value associated with the original restricted stock unit award (see footnotes 2 and 3). This restricted stock unit award was originally granted on February 1, 2018 and vests over a four-year period at a rate of 25% on 01/04/2019, 25% on 01/04/2020, 25% on 01/04/2021, and 25% on 01/04/2022.

Footnote F2

bluebird equity awards granted prior to January 1, 2021 were converted into equity awards of both bluebird and 2seventy. The number of shares underlying the converted bluebird equity awards was determined by multiplying the number of shares underlying the existing bluebird equity award by a fraction, the numerator of which is the volume-weighted average trading price of bluebird common stock (trading "regular way") on the five trading days immediately prior to the distribution date (the "bluebird Pre-Distribution VWAP") and the denominator of which is the sum of (1) the volume-weighted average trading price of 2seventy common stock (trading "regular way") on the five trading days immediately following the distribution date multiplied by the distribution ratio and (2) the volume-weighted average trading price of bluebird bio common stock (trading "regular way") on the five trading days immediately following the distribution date (the "bluebird Post-Distribution VWAP").

Footnote F3

bluebird equity awards granted on or after January 1, 2021, such equity awards continued as bluebird equity awards, subject to adjustment. The adjusted number of shares of bluebird common stock underlying such equity awards is equal to the number of shares of bluebird common stock subject to the equity award immediately prior to the distribution multiplied by a fraction, the numerator of which is the bluebird Pre-Distribution VWAP and the denominator of which is the bluebird Post-Distribution VWAP.

Footnote F4

Represents restricted stock units awarded to the reporting person prior to the Separation. In connection with the Separation, the restricted stock units underlying this award were adjusted pursuant to the EMA such that the reporting person received restricted stock units of 2seventy to preserve the value associated with the original restricted stock unit award (see footnotes 2 and 3). This restricted stock unit award was originally granted on February 1, 2019 and vests over a four-year period at a rate of 25% on 01/04/2020, 25% on 01/04/2021, 25% on 01/04/2022, and 25% on 01/04/2023 .

Footnote F5

Represents restricted stock units awarded to the reporting person prior to the Separation. In connection with the Separation, the restricted stock units underlying this award were adjusted pursuant to the EMA such that the reporting person received restricted stock units of 2seventy to preserve the value associated with the original restricted stock unit award (see footnotes 2 and 3). This restricted stock unit award was originally granted on March 2, 2020 and vests over a four-year period at the rate of 25% on 01/04/2021, 25% on 01/04/2022, 25% on 01/04/2023, and 25% on 01/04/2024.

Footnote F6

Represents restricted stock units awarded to the reporting person prior to the Separation. In connection with the Separation, the restricted stock units underlying this award were adjusted pursuant to the EMA to preserve the value associated with the original restricted stock unit award (see footnotes 2 and 3). This restricted stock unit award was originally granted on February 16, 2021 and vests over a four-year period at a rate of 25% on 01/04/2022, 25% on 01/04/2023, 25% on 01/04/2024, and 25% on 01/04/2025.

Footnote F7

Represents restricted stock units awarded to the reporting person prior to the Separation. In connection with the Separation, the restricted stock units underlying this award were adjusted pursuant to the EMA to preserve the value associated with the original restricted stock unit award (see footnotes 2 and 3). This restricted stock unit award was originally granted on August 2, 2021 and vests over a two-year period at the rate of 40% on August 2, 2022 and 60% on August 2, 2023.

Footnote F8

Represents restricted stock units awarded to the reporting person prior to the Separation. In connection with the Separation, the restricted stock units underlying this award were adjusted pursuant to the EMA to preserve the value associated with the original restricted stock unit award (see footnotes 2 and 3). This restricted stock unit award was originally granted on August 2, 2021 and vests upon the satisfaction of certain performance criteria.

Footnote F9

Represents options to purchase bluebird common stock awarded to the reporting person prior to the Separation. In connection with the Separation, the shares of common stock underlying this award were adjusted pursuant to the EMA such that the reporting person received options to purchase shares of 2seventy common stock to preserve the value associated with the original award (see footnotes 2 and 3). This option was originally granted on July 1, 2015 and is fully vested and exercisable as of the date hereof. The exercise price of the option has been adjusted based on the conversion ratio described in footnotes 2 and 3.

Footnote F10

Represents options to purchase bluebird common stock awarded to the reporting person prior to the Separation. In connection with the Separation, the shares of common stock underlying this award were adjusted pursuant to the EMA such that the reporting person received options to purchase shares of 2seventy common stock to preserve the value associated with the original award (see footnotes 2 and 3). This option was originally granted on March 1, 2016 and is fully vested and exercisable as of the date hereof. The exercise price of the option has been adjusted based on the conversion ratio described footnotes 2 and 3.

Footnote F11

Represents options to purchase bluebird common stock awarded to the reporting person prior to the Separation. In connection with the Separation, the shares of common stock underlying this award were adjusted pursuant to the EMA such that the reporting person received options to purchase shares of 2seventy common stock to preserve the value associated with the original award (see footnotes 2 and 3). This option was originally granted on February 1, 2017 and is fully vested and exercisable as of the date hereof. The exercise price of the option has been adjusted based on the conversion ratio described in footnotes 2 and 3.

Footnote F12

Represents options to purchase bluebird common stock awarded to the reporting person prior to the Separation. In connection with the Separation, the shares of common stock underlying this award were adjusted pursuant to the EMA such that the reporting person received options to purchase shares of 2seventy common stock to preserve the value associated with the original award (see footnotes 2 and 3). This option was originally granted on February 1, 2018 and vests over a four-year period, at a rate of 25% on 01/04/2019 and in 36 equal monthly installments thereafter. The exercise price of the option has been adjusted based on the conversion ratio described in footnotes 2 and 3.

Footnote F13

Represents options to purchase bluebird common stock awarded to the reporting person prior to the Separation. In connection with the Separation, the shares of common stock underlying this award were adjusted pursuant to the EMA such that the reporting person received options to purchase shares of 2seventy common stock to preserve the value associated with the original award (see footnotes 2 and 3). This option was originally granted on February 1, 2019 and vests over a four-year period, at a rate of 25% on 01/04/2020 and in 36 equal monthly installments thereafter. The exercise price of the option has been adjusted based on the conversion ratio described in footnotes 2 and 3.

Footnote F14

Represents options to purchase bluebird common stock awarded to the reporting person prior to the Separation. In connection with the Separation, the shares of common stock underlying this award were adjusted pursuant to the EMA such that the reporting person received options to purchase shares of 2seventy common stock to preserve the value associated with the original award (see footnotes 2 and 3). This option was originally granted on March 2, 2020 and vests over a four-year period, at a rate of 25% on 01/04/2021 and in 36 equal monthly installments thereafter. The exercise price of the option has been adjusted based on the conversion ratio described in footnotes 2 and 3.

Footnote F15

Represents options to purchase bluebird common stock awarded to the reporting person prior to the Separation. In connection with the Separation, the shares of common stock underlying this award were adjusted pursuant to the EMA to preserve the value associated with the original award (see footnotes 2 and 3). This option was originally granted on February 16, 2021 and vests over a four-year period, at a rate of 25% on 01/04/2022 and in 36 equal monthly installments thereafter. The exercise price of the option has been adjusted based on the conversion ratio described in footnotes 2 and 3.