Severe side effects that caused the discontinuation and reduced the duration of palbociclib and endocrine therapies were fatigue (Grade ≥ 3), decreased neutrophil counts (Grade ≥ 3), increased aspartate aminotransferase (AST) or alanine aminotransferase (ALT) levels (Grade ≥ 3), and pneumonitis (Grade ≥ 2). The gene discussed is GPT; the disease is pneumonitis.