DMS-79 is a human cell line derived from a small cell lung carcinoma (SCLC), which expresses the pro-opiomelanocortin (POMC) gene. We took it as a model in which to study the mechanism of POMC gene expression in non-pituitary tumours. DMS-79 reproduces the usual characteristics of POMC expression in these tumours: precursor processing is altered and gene expression is essentially unresponsive to glucocorticoids. POMC gene structure appeared normal by Southern blot analysis, indicating that gene rearrangement was not responsible for its expression in DMS-79. Indeed, using transient expression of human POMC—luciferase fusion genes in DMS-79, we showed that (1) the normal human POMC promoter was functional in DMS-79, and (2) the same proximal promoter region (−417;+21) produced the full transcriptional activity in DMS-79 and in the mouse pituitary cell line AtT-20. Progressive 5′ deletion analysis revealed differences between AtT-20 and DMS-79: region (−161;−376) was active in AtT-20 and not in DMS-79, whereas region (−95;−161) was active in both cell lines and (−376;−417) was only active in DMS-79. The latter partially overlaps a motif homologous to the DE-2 rat element which confers the tissue-specific expression of POMC in AtT-20 cells; however, this motif had no effect in DMS-79. These data suggest that POMC gene transcription is achieved through a different set of transacting factors in DMS-79 and AtT-20. Altogether, our results provide evidence that DMS-79 is a valid model of tumours responsible for the ectopic ACTH syndrome and that the mechanism of POMC gene expression in these SCLC cells is different from that in pituitary cells.