Abstract: The invention concerns eukaryotic cells useful in protein expression comprising (a) an inserted nucleic acid encoding a cyclin D gene product and (b) an inserted nucleic acid encoding a protein of interest, wherein the cyclin D gene product and the protein of interest are expressed in the cell. The invention also concerns a process for producing a protein of interest, which comprises (a) inserting into a eukaryotic cell a nucleic acid encoding a cyclin D gene product and a nucleic acid encoding a protein of interest; (b) culturing the cell under conditions permitting the expression of the protein of interest; and (c) isolating the protein of interest. The cells are preferably mammalian, with CHO cells most preferred. The cyclin D gene product is preferably of human origin. Suitable proteins of interest include erythropoietin (EPO), osteoprotegerin (OPG), OPG-Fc, leptin, Fc-leptin, and Novel Erythropoiesis Stimulating Protein (NESP).