The function of ITSN1 (intersectin 1, Ensembl gene identifier ENSG00000205726) is as follows. Adapter protein that provides a link between the endocytic membrane traffic and the actin assembly machinery (PubMed:11584276, PubMed:29887380). Acts as a guanine nucleotide exchange factor (GEF) for CDC42, and thereby stimulates actin nucleation mediated by WASL and the ARP2/3 complex (PubMed:11584276). Plays a role in the assembly and maturation of clathrin-coated vesicles (By similarity). Recruits FCHSD2 to clathrin-coated pits (PubMed:29887380). Involved in endocytosis of activated EGFR, and probably also other growth factor receptors (By similarity). Involved in endocytosis of integrin beta-1 (ITGB1) and transferrin receptor (TFR); internalization of ITGB1 as DAB2-dependent cargo but not TFR may involve association with DAB2 (PubMed:22648170). Promotes ubiquitination and subsequent degradation of EGFR, and thereby contributes to the down-regulation of EGFR-dependent signaling pathways. In chromaffin cells, required for normal exocytosis of catecholamines. Required for rapid replenishment of release-ready synaptic vesicles at presynaptic active zones (By similarity). Inhibits ARHGAP31 activity toward RAC1 (PubMed:11744688). {ECO:0000250|UniProtKB:Q9WVE9, ECO:0000250|UniProtKB:Q9Z0R4, ECO:0000269|PubMed:11584276, ECO:0000269|PubMed:11744688, ECO:0000269|PubMed:22648170, ECO:0000269|PubMed:29887380}. [Isoform 1]: Plays a role in synaptic vesicle endocytosis in brain neurons. .