The Ink4a/Arf locus encodes two distinct proteins, both of which may contribute to senescence and tumor suppression. We find that human diploid fibroblasts (HDFs) that are specifically deficient for p16INK4a achieve anchorage independence when transduced with retroviruses encoding telomerase (hTERT) and either Ras or Myc. Significantly, Ras and Myc together enable the cells to form tumors in nude mice but at a frequency that suggests additional genetic changes. All five tumors analyzed expressed high levels of Ras and retained functional p53, although two showed downregulation of Arf. Cytogenetic analyses identified clonal chromosomal alterations that may have contributed to tumorigenesis, but the tumor cells were essentially diploid.