The company, based in Emeryville near San Francisco, said the U.S. Patent and Trademark Office granted it a patent for a genetically engineered protein with the potential to treat several types of cancer and infectious disease, including AIDS. The human body's immune system naturally produces the protein, called interleukin-2. Cetus' version of it differs slightly from the form found in nature. The patent is the first for a protein altered by genetic engineering and intended as a drug. Cetus hopes to market the drug by 1988.