The human c-myc proto-oncogene contains an open reading frame within its first exon which is translated into protein (MYCHEX1). While the murine c-myc exon 1 is obviously non coding, we show that in mouse cells there are polypeptides closely related to human MYCHEX1. These polypeptides share the same immunological reactivities with the human polypeptides. Furthermore, the 32 kDa polypeptide of murine cells has, like its human counterpart, the ability to dimerise in a 58 kDa form in denaturing and reducing SDS-PAGE. The human gene was introduced into a murine cell line by transfection. A cell line was studied, in which the inducible expression of the gene allows a substantial increase in the concentration of the corresponding protein. This inducible protein behaves in any respect like the murine one, either in SDS-PAGE or in a specific immunoassay. These shared properties constitute a further proof that the human and mouse MYCHEX1 proteins are encoded by the sequence overlapping the human myc exon 1 and a related murine sequence. The gene contained in the human c-myc exon 1 is not, therefore, a specific feature of human cells.