The Axl receptor tyrosine kinase is a transforming oncogene in NIH3T3 cells. In order to define structural requirements of the Axl receptor necessary for transformation we passaged recombinant retroviruses carrying the axl cDNA in NIH3T3 cells, generating randomly mutated axl variants. Using this strategy, we have isolated three axl viral strains (1B1, SV8, and FFa4) that show augmented 3T3 cell transforming capacity associated with elevated p140Axl. Upon sequencing, the 1B1 and SV8 proviruses possessed only silent mutations, making p140Axl overexpression the most likely explanation for their increased transformation activity. However, the characterization of FFa4 revealed a deletion of sequences encoding the carboxy-terminal 45 amino acids leading to the generation of a chimeric transcript comprised of a truncated Axl receptor with a segment of the 3' UTR region. Mutational analysis revealed that the transforming activity of FFa4 was specific to the formation of the chimeric receptor rather than to the carboxyl-terminal truncation. Intriguingly, none of the viral strains were able to transform the murine cell lines NR-6 and 32D despite equivalent expression of surface p140Axl protein. Further analysis showed that Axl's transforming potential is dependent on the host cell type, the presence of a putative pp190 as a facilitator for transformation, and the level of p140Axl expression. Taken together, these results underscore the complexity of Axl biology which is dependent on receptor stoichiometry and the cellular background.