GCK and breast cancer: HK1 is constitutively expressed in multiple tissues; HK2 is expressed in embryonic tissue and aggressive tumors, such as lung cancer, hepatocyte cell cancer, breast cancer, and prostate cancer2–4; HK3 and HK5 (known as HKDC1) are poorly characterized5; HK4, also known as glucokinase, is located primarily in the liver and the endocrine pancreas.