Spark Therapeutics Inc., a Philadelphia gene-therapy developer, would be bought by Switzerland-based Roche Holding AG in a deal said to be close and possibly valued at almost $5 billion, the Wall Street Journal reported Saturday, citing unnamed knowledgeable sources.

The Journal said the deal could be announced Monday, if not sooner. Spokespeople for Spark and Roche have declined to comment.

Spark, founded in 2013 by two researchers at Children’s Hospital of Philadelphia and public since 2015, had a market capitalization of $1.95 billion on Friday. Roche, with a market cap of $230 billion, faces one other bidder for Spark, the Wall Street Journal reported, without disclosing who that is.

Spark, a pioneer in gene therapy for genetic disease, has a portfolio that includes work on new drugs for the bleeding disorder hemophilia, and therapies for Huntington’s disease, a fatal genetic brain disorder, and Batten’s disease, a fatal nervous-system condition -- potentially multibillion-dollar opportunities in an emerging market. Unique competencies in discovery, development, and delivery “increase our ability to successfully break barriers by transforming the research in our labs into life-altering gene therapy products ... and position us well for continued growth and success,” CEO Jeffrey D. Marrazzo said earlier this month in announcing the company’s 2018 financial results and business progress.

Word of a possible deal provided another jolt of excitement for the University City Science Center, where Spark manufactures and where another resident, the biotech start-up Invisible Sentinel, a specialist in food safety, announced its sale earlier this month for $75 million to France-based BioMérieux, a premiere player in diagnosing infectious diseases worldwide.

The Roche deal described by the Wall Street Journal would represent a “very strong vote of confidence by Roche in Spark’s product pipeline and expertise,” said Stephen Tang, the Science Center’s former president and CEO, and now head of Bethlehem-based OraSure Technologies Inc., developers of medical diagnostic kits.

In a city hoping to parlay its health-related research into a reputation as a place where biotech start-ups can thrive and achieve high-value exits, a $5 billion valuation of Sparks would be “a remarkable exit for this revolutionary company,” Tang said.

Spark, in which CHOP remains a minority stakeholder, reported nearly $65 million in total revenue for 2018, of which $27 million came from net sales of Luxturna. In December 2017, Luxturna became the first gene therapy approved for a genetic disease by the U.S. Food and Drug Administration. Used to treat Leber’s congenital amaurosis, a rare inherited form of blindness that affects the retinas of about 1,000 children in the United States, a single injection of Luxturna to the retina restores sight in infants.

In January 2018, the Swiss drugmaker Novartis agreed to pay Spark $105 million, and up to $65 million more pending European regulatory approvals and early sales, to sell Luxturna outside the U.S.

Last April, Spark, announced it would receive a $110 million jackpot it planned to use to fund research and development on new experimental drugs in its pipeline for hemophilia. Roche’s pipeline also includes drugs for hemophilia, the company has reported.

That windfall came from Spark’s selling a “priority review voucher” to the Irish biopharmaceutical company Jazz Pharmaceuticals, which has offices in Philadelphia. The FDA uses such vouchers -- which fast-track federal review of medicines -- to encourage drug companies to develop treatments for rare illnesses in children and tropical diseases. Spark had received the voucher it sold to Jazz in December 2017 after the FDA approved Luxturna.

Roche Holding’s stock closed up Friday at $34.73 a share; Spark closed up at $51.56 a share.