STAT5B and cancer: Even though gain‐of‐function (GOF) mutations of STAT5B were reported in a minor fraction (3.6%) of CTCL patients (Park et al, 2017), our WES data were negative for GOF JAK–STAT mutations, but we consistently found copy number gains of STAT3/5 representing a cancer genome hallmark for the majority of L‐CTCL patients.