ESR1 and breast cancer: In 2016, a phase III, randomized PALOMA-2 study in postmenopausal women with ER-positive/HER2-negatvie advanced breast cancer further revealed that infection (any preferred term under the System Organ Class Infections and Infestations) was the most common serious AE among both Asian and non-Asian patients in the palbociclib plus letrozole group (Im et al., 2019).