MCU-i4 decreases mitochondrial Ca2+ influx. Docking simulations reveal that MCU-i4 directly binds a specific cleft in MICU1, a key element of the MCU complex that controls channel gating. Accordingly, in MICU1-silenced or deleted cells, the inhibitory effect of MCU-i4 is lost. Moreover, MCU-i4 fails to inhibit mitochondrial Ca2+ uptake in cells expressing a MICU1 mutated in the critical amino acids that forge the predicted binding cleft.