The deal closure comes nine months after it was first announced - Genentech and Tanox, a Texas-based biotech company, had to go through a waiting period under the Hart-Scott-Rodino Act which has now expired.
The Hart-Scott-Rodino Act is a federal law forbidding businesses from monopolising the market.
It requires that any company seeking to acquire either a 15 per cent stake or over $15m, in another company, has to file a notification with the federal government.
Tanox shareholders approved the merger in Juanuary and will receive £20 in cash per share of common stock, which will no longer being traded on the Nasdaq market.
This move gives Genentech access to a part of the commercial rights for Xolair (omalizumab), an anti-IgE monoclonal antibody (mAb) for patients with asthma it markets with Novartis, which was actually developed by Tanox.
Genentech, Tanox and Novartis have collaborated on the development and commercialisation of Xolair since 1996.
With the closing of the acquisition, Genentech saves on Xolair sales royalties which it previously paid to Tanox and also obtains Novartis' profit share and royalty payments to Tanox.
In total, Tanox received more than $40m in royalty payments for the drug in 2006, but Genentech would not disclose what its share was.
Xolair was approved in 2003 in the US as the first biotechnology treatment for moderate-to-severe allergic asthma.
Sales of the drug in the US reached $425m in 2006, of which $140m went to Novartis.
The drug is also available in 20 EU countries since being approved in October 2005 and generated $102m in sales last year in the region.
Genentech said the closing of the deal "will result in an improvement of our financial results for Xolair and the acquisition of Tanox's product pipeline, which has some interesting molecules being developed for diseases such as asthma, HIV and age-related macular degeneration (AMD)."
"We are currently evaluating Tanox' pipeline to select which products could have a place in our portfolio," a Genentech spokesperson told BioPharma-Reporter.com.
She said the company was particularly interested in Tanox's TNS-650 - an mAb tested to treat asthma and currently in Phase I - and TNX-234, another antibody tested for the treatment of wet AMD, currently at the preclinical stage.
In addition, Genentech gains access to more advanced compound, TNX-355 - a viral-entry inhibitor antibody to treat AIDS - which has shown positive results in Phase II clinical trials.
But first, the company will focus on exploiting further the potential of Xolair.
"We will continue to work with our collaborator Novartis to advance potential new indications and formulations of Xolair," said David Ebersman, executive vice president and chief financial officer of Genentech.