NRXN1 Full-Length MS Protein Standard (NP_004792), Labeled with [U- 13C6, 15N4]-L-Arginine and [U- 13C6, 15N2]-L-Lysine, was produced in human 293 cells (HEK293) with fully chemically defined cell culture medium to obtain incorporation efficiency at Creative-Proteomics. Neurexins function in the vertebrate nervous system as cell adhesion molecules and receptors. Two neurexin genes are among the largest known in human (NRXN1 and NRXN3). By using alternate promoters, splice sites and exons, predictions of hundreds or even thousands of distinct mRNAs have been made. Most transcripts use the upstream promoter and encode alpha-neurexin isoforms; fewer transcripts are produced from the downstream promoter and encode beta-neurexin isoforms. Alpha-neurexins contain epidermal growth factor-like (EGF-like) sequences and laminin G domains, and they interact with neurexophilins. Beta-neurexins lack EGF-like sequences and contain fewer laminin G domains than alpha-neurexins. The RefSeq Project has decided to create only a few representative transcript variants of the multitude that are possible.