Negative regulator of B-cell activation. Down-regulates cell proliferation (in vitro). Promotes RAC1-dependent membrane ruffle formation and reorganization of the actin cytoskeleton. Regulates cell spreading and cell polarization. Stimulates HDAC1 activity. Regulates LYN activity by modulating its tyrosine phosphorylation (By similarity). . This is the function of SAMSN1 (SAM domain, SH3 domain and nuclear localization signals 1, Ensembl gene identifier ENSG00000155307).