Rash, possibly due to the expression of glycoprotein non-metastatic melanoma protein B (GPNMB) in healthy skin, was the most common toxicity reported after treatment with glembatumumab, another ADC containing monomethyl auristatin E that is being evaluated for the treatment of GPNMB-expressing breast cancers and melanomas [30]. The gene discussed is GPNMB; the disease is breast cancer.