Verapamil (VPM) as P-gp-inhibiting drug significantly improved MM treatment response in drug-resistant MM patients in the pre-PI era.26, 27 Although CFZ is a bona fide substrate for ABCB1,18 conflicting data exist for BTZ.28, 29 The functional role of ABCB1 overexpression for PI resistance of MM and its implications for the use of, or choice between, different PI drugs are unknown. Here, PGP is linked to Miyoshi myopathy.