Isoform: A protein that has the same function as another protein but which is encoded by a different gene and may have small differences in its sequence. For example, transforming factor beta (TGF-B) exists in three versions, or isoforms (TGF-B1, TGF-B2, and TGF-B3), each of which can set off a signaling cascade that starts in the cytoplasm and terminates in the nucleus of the cell.
