In contrast to normal cells where Thr 163 phosphorylation can promote GSK3/Ser 159 phosphodegron-targeted Mcl-1 degradation and death, in cancer cells in which Mcl-1 is not degraded through this pathway, ERK activation and Thr 163 phosphorylation are associated with reduced Mcl-1 degradation and striking drug resistance. Here, MCL1 is linked to cancer.