Cytogenetic analysis of eight ultraviolet radiation (UVR)-induced cutaneous tumors and one spontaneously transformed fibroblast cell line of Monodelphis domestica showed that two of the tumor cell lines were of murine origin and that the remaining six marsupial tumor cell lines had hyperdiploid stemline numbers ranging from 21 to 31. Each tumor cell line showed structural and numerical abnormalities. The single transformed fibroblast cell line also showed a hyperdiploid chromosome number with structural and numerical defects. All M. domestica tumor cell lines and the fibroblast line showed a common structural abnormality: deletion of the short arm (p) of a chromosome 1. In some cell lines, the short arm of chromosome 1 was replaced by a translocation with the X chromosome. We suggest, based on the Giemsa-banding homology of chromosome 1p in M. domestica and human chromosome 6q involved in melanomas, that marsupial chromosome 1p may harbor tumor suppressor gene(s) that are associated with UVR-induced cutaneous tumors.