Updated from 8:45 a.m. EDT
slipped Monday despite positive test data on a cancer drug the two companies are developing.
Data presented at the American Society of Oncology's annual meeting Saturday showed the companies' Tarceva treatment increased survival by 42% in patients with end-stage non-small-cell lung cancer.
Shares in both companies rose in early trading, but then fell back. Genentech shares fell $3.10, or 5.2%, to $56.31 while OSI shares lost $4.51, or 5.7%, to $74.02.
"This trial is important because a survival treatment benefit was achieved among a broad group of patients," said Genentech's chief medical officer of the phase III testing.
OSI is expected to seek marketing approval for Tarceva from the Food and Drug Administration this summer.
will market the drug overseas if and when it is approved.
The companies said "more detailed presentations of the data" will be made at the society's meeting today.