Erythropoietin (EPO) regulates mammalian erythropoiesis by binding to its transmembrane receptor EPOR. Recent studies demonstrated functional EPOR expression in human cancer cells. Recombinant human EPO was reported to stimulate the proliferation of monolayer cultures of breast and renal carcinoma cells. Furthermore, administration of EPO-EPOR antagonists delayed the growth of uterine, ovarian, and mammary carcinoma cells in experimental animal models. In this study, we show EPOR transcript and protein expression in breast, colon, lung, ovary, and prostate cancer cells. Using reverse transcription-polymerase chain reaction. we isolated and characterized several novel cDNAs for EPOR splice variants expressed in cancer cells. Deduced amino acid sequences of the cDNAs revealed splice variants encoding soluble EPOR or membrane-bound EPOR peptides with intra-cytoplasmic, carboxy-terminal truncations. These findings indicate the expression of multiple EPOR isoforms in human cancer cells that may modulate the cellular effects of recombinant human EPO or EPO-EPOR antagonists. (C) 2003 Elsevier Inc. All rights reserved.