Acts as a negative regulator of SRC by activating CSK which inhibits SRC activity and downstream signaling, leading to impaired cell spreading and migration. Regulates dendritic spine morphology. Involved in calcium-dependent exocytosis. May play a role in neurotransmitter release or synapse maintenance. {ECO:0000269|PubMed:14657239, ECO:0000269|PubMed:17525734, ECO:0000269|PubMed:19146815}. This is the function of SRCIN1 (SRC kinase signaling inhibitor 1, ENSG00000277363).