As treatment with MCL-1 targeting BH3-mimetic drugs induced apoptosis of breast cancer cells in vitro we tested the in vivo potential of such drugs. To clinically model breast cancer treatment we commenced pharmaceutical intervention once xenograft tumours had become clinically detectable. To this end, MDA-MB-468 breast cancer cells were injected into the mammary fat pads of BALB/cNude mice. Tumours were allowed to establish and when they reached ~5mm diameter, treatment with the MCL1-specific inhibitor UMI-77 (60mg/kg) or vehicle control commenced by intraperitoneal injection 5 times per week. Treatment with UMI-77 significantly delayed growth of established MDA-MB-468 xenografts (Fig. 4a).