Most known arthritis-linked genes and loci associate with both PIA and CIA (Table 1), including the RT1-B (Haag et al., 2015; Tuncel et al., 2012), Ncf1 (Olofsson et al., 2003c) and APLEC (Bäckdahl et al., 2003; Lorentzen et al., 2007; Rintisch et al., 2010) genes discussed in this Review. Here, NCF1 is linked to arthritic joint disease.