The deal for Spark could be announced Monday, if not sooner, at a price tag of nearly $5 billion (£3.8 billion), but could still fall through, the Journal said, citing the unidentified sources. Spark's market value was just under $2 billion at Friday's market close.

The newspaper reported there was at least one other unidentified bidder for Spark as of Friday.

Officials with Roche and Spark said their companies do not comment on market rumours.

Spark is a gene therapy company with a pipeline of potential products targeting blindness, haemophilia and neurodegenerative diseases.

(Reporting by Ben Klayman in Detroit; Editing by Paul Simao)