Genentech (NYSE:DNA) released second-quarter earnings yesterday, with the company crossing the milestone of $1 billion in quarterly revenue for the first time. Driving the growth is new cancer drug Avastin, with an impressive $133 million in sales in its first full quarter on the market since it was approved in late February. The market had exceptionally high expectations for this product, and it certainly seems well on its way to becoming a blockbuster drug.

For some perspective on the Avastin sales number, consider that Genentech and Biogen Idec's (NASDAQ:BIIB) star drug Rituxan had under $40 million in sales in its first quarter. Seven years after approval, Rituxan is now looking like it will become a $2 billion drug. Genentech's other big cancer drug, Herceptin, also didn't break $40 million in sales during its first quarter.

The reason for Avastin's exceptional sales ramp is rapid uptake into the colorectal cancer market. It was discussed in yesterday's conference call that Avastin has 20% penetration in front-line therapy and greater than 30% penetration as second-line treatment in patients with relapsed or refractory disease. Those are impressive figures for a drug that was approved only five months ago.

A fast drug launch usually indicates peak sales in the future will be substantial. How high can Avastin sales go? Given the market-share figures in colorectal cancer, I think it is very likely that there is room for sales to at least double in that disease. That alone makes Avastin a billion-dollar product. Over time, with expansion into other markets, Avastin could rival Rituxan as Genentech's top product.

For additional articles on Genentech, see:

Genentech's Unique Drug

Genentech's Lucky Day

Genentech's Healthy Showing

Genentech Priced For Perfection

Fool contributor Charly Travers does not own shares of any company mentioned in this article.