As mortalin is mainly located in mitochondria in MTC cells [15], we speculate that mitochondrial partitioning is a key criterion for the potency of the MKT-077 derivatives in MTC cells, although other factors, including import/export kinetics, ‘off-target’ effects, and other as-yet-unidentified mechanisms may also be involved. Here, HSPA9 is linked to medullary thyroid gland carcinoma.