Many studies have established the role of SPRR1B during squamous differentiation of skin and respiratory epithelial cells. However, its role in non-squamous cells is largely unknown. We had reported that expression of SPRR1B in Chinese hamster ovary (CHO) cells is increased as they enter the G0 phase of the cell cycle. The purpose of this study was to further investigate the SPRR1B expression pattern in non-squamous tumors and to study its role in these cells. Expression of SPRR1B was detected by Northern blotting in a higher percentage of NNK-induced compared to beryllium metal-induced rat lung adenocarcinomas. In situ hybridizations confirmed that SPRR1B was expressed in individual or clusters of cells of non-squamous cells from mouse, rat, and human adenocarinomas. The same pattern of expression was observed in adenocarcinomas formed in nude mice from cell lines established from adenocarcinomas. SPRR1B expression was downregulated in the cell lines derived from adenocarcinoma when cells were enriched in G0 at low confluence. Tetraploidy was induced in CHO, mouse, and human tumor cell lines by stably overexpressing SPRR1B, while control cells showed no change in ploidy. Inducible expression of this protein for short periods using the ecdyson system did not affect growth rate or the ploidy of CHO cells, but accelerated entry into G0/G1 compared to controls. These findings indicate that SPRR1B is likely coupled primarily to signals responsible for withdrawal from the proliferative state rather than the final stages of cellular quiescence and that its overexpression for prolonged periods may disrupt the normal progression of mitosis.