Oncogenes have been implicated in tumorigenesis based on their localization to chromosomal sites associated with tumor-specific structural rearrangements. We have mapped the human ros1 (formerly mcf3) and mas1 oncogenes to the distal half of chromosome 6q, within a region frequently rearranged in malignant cells. Chromosomal mapping of these two new human transforming genes may help elucidate the involvement of the long arm of chromosome 6 in diverse tumor types.