Pseudotypes of feline sarcoma virus contain an 85,000-dalton protein with feline oncornavirus-associated cell membrane antigen (FOCMA) activity.

Feline sarcoma virus (FeSV) rescued from transformed nonproducer mink or rat cells contains two FeSV-specific antigens (p15 and p12), and the feline oncornavirus-associated cell membrane antigen (FOCMA). All three antigens are helper virus-independent and are encoded by the FeSV genome, FOCMA, p15, and p12 antigens cochromatograph as phosphorylated molecules of 85,000 molecular weight (pp85), adsorb to immunoadsorbant columns prepared with antibodies to feline leukemia virus (FeLV), and are precipitated with antisera to FeLV or FOCMA. Antibodies to FOCMA can be adsorbed with fractions containing pp85 but not with FeLV proteins, including p15 and p12. Thus, a virus-coded tumor antigen which immunizes cats against tumors induced by feline type C viruses is packaged in FeSV particles and is linked to viral structural protein.