Since May 19, the day Genentech ( DNA) said its experimental drug Avastin improved the survival of patients with colon cancer when used with chemotherapy, the company's shares have been on a tear, rising more than 60%.

CIBC World Markets has seen enough for now. The firm downgraded shares of Genentech Thursday to sector perform from sector outperform. The company's market capitalization has expanded by around $12 billion since the Avastin results were released, and CIBC said in a research note that it believes the move reflects $1.5 billion to $2 billion in peak Avastin sales.

The downgrade was a valuation call, and CIBC still wrote positively about Genentech and Avastin in general. The new rating means that CIBC expects Genentech to perform in line with the biotechnology group for the next year to year and a half.

"Based solely on current information, there is no clear upside, in our opinion," CIBC wrote in a research note. "We still regard Genentech highly, but with the recent appreciation, we believe there are better short- to intermediate-term opportunities."

The firm also said it is no longer carrying a price target on the stock. CIBC previously had a 12- to 18-month target of $53.

CIBC said it was optimistic about Avastin's use in treating renal cell carcinoma. The firm also said it believes the data Genentech will present this weekend at the American Society of Clinical Oncology meeting for metastatic colon cancer will be "very positive."

The firm said questions still remain about the drug's effectiveness against nonsmall-cell lung cancer and other cancers. But CIBC said it does believe Avastin will be approved for the front-line treatment of metastatic colorectal cancer, an estimated $800 million to $1 billion market opportunity.

Shares of Genentech were lately down $2.67, or 4%, to $60.20. The Amex Biotech Index was unchanged, and the Nasdaq Biotech Index was gaining 0.9%.

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