TK1 and breast cancer: TK exists in two forms: TK1 is found primarily in the cytoplasm, and TK2 is concentrated in mitochondria.23 Since TK1 is cell‐cycle regulated and TK2 is constitutively expressed, the value of sTK1 as a biomarker for diagnosis and its prognostic significance in lung cancer, breast cancer, esophageal cancer, and gastric cancer have been investigated in recent years,24, 25, 26 and our study analyzed the TK1 concentration in pleural effusion for the first time.