FKBP2 and systemic lupus erythematosus: To determine whether FKBP13 is a general marker of long-lived PCs, we carried out the same assays with F1 female hybrids of NZB and NZW strains (referred to hereafter as NZB/W F1), a canonical model of lupus and also known to have a substantial number of long-lived PCs in the spleen (SP) (34).