May 14 (Reuters) - Bluebird bio and Carlyle

Group ( CG ) and SK Capital Partners said on Wednesday that the

private equity firms have offered $5 per share upfront to

takeover the gene therapy maker.

Shareholders may elect to receive either the new offer or

the previous offer of $3 per share, with an additional $6.84

contingent upon bluebird achieving certain sales milestones.

(Reporting by Puyaan Singh in Bengaluru' Editing by Anil

D'Silva)