Molecular analysis of the human proto-oncogene L-myc revealed a complex pattern of gene expression including alternative splicing and polyadenylation site selection of mRNA, giving rise to at least four mRNAs. These mRNAs in turn can code for several proteins. In this report, we characterize and define the origins of the major L-myc proteins. In vitro translation revealed that (i) two L-myc proteins (p59 and p65) were derived through alternative translational initiation at a non-AUG (CUG) site in intron 1 and at an AUG site in exon 2 of L-myc, and that (ii) extensive post-translational phosphorylation of these proteins yielded three additional proteins (p60, p66, and p68). Transfection experiments in rat embryo cells revealed the in vivo existence of this unusual CUG-initiated protein and demonstrated that it possessed transforming activity. Further, immunoprecipitation using high titered anti-L-myc peptide antisera, of two L-myc expressing small-cell lung cancer cell lines revealed three major L-myc proteins (p60, p66 and p68) all of which were derived from extensive phosphorylation of a p59 protein.