We have cloned and sequenced a new human oncogene and have named it mas. This oncogene was detected by its tumorigenicity in nude mice using the cotransfection and tumorigenicity assay previously described. The mas oncogene has a weak focus-inducing activity in transfected NIH 3T3 cells. A DNA rearrangement in the 5' noncoding sequence, which occurred during transfection, is probably responsible for activation of the mas gene. The cDNA sequence of the mas oncogene reveals a long open reading frame that codes for a 325 amino acid protein. This protein is very hydrophobic and has seven potential transmembrane domains. In this respect, the structure of the mas protein is novel among cellular oncogene products and may reflect a new functional class of oncogenes.