GADD45B and Miyoshi myopathy: Significantly, z-DTP1 and z-DTP2, but not a control D-tetrapeptide, induced potent and dose-dependent toxicity in all of the MM cell lines tested, except the two expressing nearly undetectable levels of GADD45B and low levels of MKK7 (further discussed below), exhibiting IC50 values in the sensitive MM cell lines in the low nanomolar to low micromolar range (Figures 4A and 4B; Figures S2G, S4E, and S4F).