Disclosed are immortalized human embryonic retina cells, having a nucleic acid sequence encoding an adenoviral E1A protein integrated into the genome of the cells, and further comprising a nucleic acid sequence encoding an enzyme involved in post-translational modification of proteins, such as a sialyltransferase, wherein said nucleic acid sequence encoding the enzyme involved in post-translational modification of proteins is under control of a heterologous promoter. Methods for producing recombinant proteins from such cells and obtaining such recombinant proteins having increased sialylation are provided as are novel compositions of isoforms of erythropoietin.