The function of ARHGAP21 (Rho GTPase activating protein 21, Ensembl gene identifier ENSG00000107863) is as follows. Functions as a GTPase-activating protein (GAP) for RHOA and CDC42. Downstream partner of ARF1 which may control Golgi apparatus structure and function. Also required for CTNNA1 recruitment to adherens junctions. {ECO:0000269|PubMed:15793564, ECO:0000269|PubMed:16184169}.