The Wall Street Journal said the acquisition could be announced by Monday, but there was no word of a sale by Sunday afternoon. Both Roche and Spark declined to offer insight in comments provided on Saturday to TheStreet.
"As a matter of corporate policy, we do not comment on market rumors or speculations," spokesman Simone Oeschger told TheStreet in an email.
A spokeswoman from Spark also said the company doesn't "comment on rumors."
The reported price would be nearly $3 billion over Spark's market cap of $1.95 billion as of Friday's close.
The Journal also said there were rumors another unnamed company could by eyeing Spark, too.
Founded about five years ago out of research from the Philadelphia Children's Hospital, Spark Therapeutics' main drug is a therapy marketed by Novartis in Europe for treatment of an inherited retinal disease, but it is also working on treatments for hemophilia. The Journal reports that Roche is targeting Spark for its work in the blood disorder area.
Spark is also working on gene therapies for other inherited diseases, including blindness, lysosomal storage disorders and neurodegenerative diseases, with three programs in clinical trials.
This story has been updated.