Platelet-derived endothelial cell growth factor/thymidine phosphorylase (PD-ECGF/TP) is a 90 kDa protein consisting of two non-covalently associated subunits. In addition to the previously identified 1.8 kilobase (kb) PD-ECGF/TP mRNA, the human epidermoid carcinoma cell line A431 was found to express 3.0 kb and 3.2 kb transcripts. cDNA cloning of the larger transcripts revealed that they contain long 5' leader sequences of 1.5 kb and 1.7 kb, respectively, and the same open reading frame encoding PD-ECGF/TP as that of the 1.8 kb transcript. Comparison with the published PD-ECGF/TP genomic sequence shows that the long 5' leader sequence contain 7 of 8 copies of Sp-1 binding sites present in the transcription promoter region of the 1.8 kb transcript.