The anti-apoptotic BCL-2 family member, Myeloid cell leukaemia-1 (MCL-1), is overexpressed in a number of cancers and provides a mechanism for drug resistance in response to BCL-2 inhibition.26 Prior studies classified the HRK BH3 domain as BCL-xL selective.17,27 To assess the selectivity of HRK constrained peptides we tested the selectivity using MCL-1. This evidence concerns the gene MCL1 and cancer.