Rac1: ProductsRas-related C3 Botulinum Toxin Substrate 1 (Rac1) is a widely expressed member of the Ras superfamily of small GTP âbinding proteins. It is included in a subfamily of Ras-related proteins that are botulinum toxin substrates. Two isoforms of Rac1 have been identified. Rac1A is a 192 amino acid (aa) propeptide that is processed by cleavage of three C-terminal aa into the 189 aa mature form, which has a predicted molecular mass of 21 kDa. The B isoform is 211 aa in length with a predicted molecular mass of 23 kDa and includes an additional 19 aa peptide sequence inserted following aa 75. The mouse and rat Rac1 orthologs share 100% aa identity with the human protein. In its activated state, Rac1 binds to a variety of effector proteins to regulate multiple cellular processes including Actin reorganization. The enzymatic activity of Rac1 is regulated by guanine nucleotide exchange factors (GEFs), GTP activating proteins (GAPs), and GDP disassociation inhibitors. Significantly, the B isoform, which has enhanced GTP/GDP exchange rates, is highly expressed in breast, lung, and colorectal cancer where it is thought to contribute to the development of neoplastic lesions.