Although the maximal plasma concentration of the catabolite mertansine is low (≤7.2 ± 2.7 nM) in T-DM1 treated cancer patients [16,17,18,19], a clinical evaluation of DDIs regarding the reduced mRNA levels by repeated treatment of T-DM1 and the CYP1A2, CYP2B6, CYP2C8/9/19, CYP3A4, UGT1A1, and UGT1A9 substrates may be necessary on the basis of these in vitro findings. The gene discussed is CYP1A2; the disease is cancer.