FKBP4 and plasma cell myeloma: Other FKBPs with relevant affinities to FK506 are FKBP13 (FKBP2) (Ki = 302 nM), FKBP51 (FKBP5) (Ki = 406 nM) and FKBP52 (FKBP4) (Ki = 56 nM) [36], all of which are expressed in myeloma cell lines.