FOLR1 and cancer: Mirvetuximab soravtansine (IMGN853) is an antibody-drug immunoconjugate that consists of a humanized monoclonal antibody (M9346A) targeting folate receptor alpha (FRα)-positive cancer cells [4] attached to a highly potent cytotoxic maytansinoid, DM4 (Fig. 1) [5].