Eleven pro-1 homologous clones from human nasopharyngeal carcinoma cell line CNE2 were studied with respect to their ability to transfer sensitivity to tumor promoter-induced neoplastic transformation (P+ activity), their molecular structure, and their homology to both mouse pro-1 and each other. Restriction mapping and Southern analysis of CNE2 pro-1 homologous clones revealed three structural classes, all of which showed P+ activity. The approximate limits of mouse pro-1 hybridizing sequences within CNE2 clones from each structural class were identified, and some of these minimum homologous sequences were tested for P+ activity by transfection into P- cells. For all classes, a strong association between P+ activity and pro-1 homology was observed. This finding implies that, for CNE2 clones to be P+-active, structural homology to mouse pro-1 is required. The minimum P+-active sequence thus far identified was a 2.6-kbp EcoRI-SstI fragment. One clone was inactive, even though it was indistinguishable from active clones of the same class by restriction mapping, Southern analysis, and electron microscope examination of heteroduplexes formed by an active and an inactive clone. This raises the possibility that discrete changes, involving a few nucleotides rather than gross rearrangement, may determine the P+ activation of these pro-1-homologous sequences.