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Shares of Bluebird Bio (BLUE) - Get bluebird bio Inc. Report  on Monday are down 3.3% after analysts at William Blair downgraded the biotech company to market perform from outperform. 

"We are taking a conservative view on the company's transition from a development-stage company to a commercial-stage company," analyst Raju Prasad wrote.

Prasad expects sales of the gene-therapy treatment Zynteglo to increase slowly as Bluebird Bio makes the transition.

The downgrade comes more than a week after the Cambridge, Mass., company reported a second-quarter net loss of $3.55 a share, wider than Zack's consensus estimate of $3.03.

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But the company also said it expected to initiate a rolling biologics licensing application submission to the FDA for Zynteglo in certain patients by the end of 2019.