Ramy Ibrahim - 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") 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 January 7, 2021 and vests ratably over three years in annual installments commencing on January 7, 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 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 June 15, 2021 and vests 100% on the earlier of June 15, 2022 or the date of the next annual meeting of stockholders.

Footnote F5

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 January 7, 2021 and vests ratably over three years in annual installments commencing on January 4, 2022. The exercise price of the option has been adjusted based on the conversion ratio described in footnotes 2 and 3.

Footnote F6

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 June 15, 2021 and vests and vests 100% on the earlier of June 15, 2022 or the date of the next annual meeting of stockholders. The exercise price of the option has been adjusted based on the conversion ratio described in footnotes 2 and 3.