The B-raf gene is the human homolog of the avian c-Rmil proto-oncogene encoding a 94-kDa serine/threonine kinase detected in avian cells. We have previously shown that this protein contains amino-terminal sequences not found in other proteins of the mil/raf gene family. These sequences are encoded by three exons in the avian genome. We report that these three exons are conserved in the human B-raf gene and that they encode an amino acid sequence similar to that of the avian c-Rmil gene, indicating that in both avian and mammalian species the product of the B-raf/c-Rmil gene is a 94-kDa protein. We also identified two human B-raf loci: B-raf-1, located on chromosome 7q34, which encodes the functional B-raf/Rmil gene product, and B-raf-2, an inactive processed pseudogene located on chromosome Xq13.