In ER+ breast cancer cells, Mcl-1 levels were induced upon treatment with the Bcl-2/Bcl-xL inhibitor ABT-263, whereas MCL1 silencing did not increase Bcl-2 or Bcl-xL, pointing to Mcl-1 as a driver of ABT-263 resistance. The gene discussed is ESR1; the disease is breast carcinoma.