Many genes are known to have several promoters. The contribution of alternative promoters to the structural and functional diversity of protein isoforms in eukaryotic cells is considered, including their role in synthesis of identical proteins from different mRNAs, generation of protein isoforms with different and even opposite functions, expression of housekeeping genes, and the variation of the recognition domains of adhesion proteins and receptors. In some cases, alternative promoters allow one gene to produce mRNAs with different open reading frames and, consequently, proteins with no amino acid sequence homology.