The function of SIPA1 (signal-induced proliferation-associated 1, ENSG00000213445) is as follows. GTPase activator for the nuclear Ras-related regulatory proteins Rap1 and Rap2 in vitro, converting them to the putatively inactive GDP-bound state (PubMed:9346962). Affects cell cycle progression (By similarity). {ECO:0000250|UniProtKB:P46062, ECO:0000269|PubMed:9346962}.