MC1R and breast cancer: The low MC1R expression level presents significant challenges for molecular imaging of human melanoma compared to other cancer imaging markers such as the prostate-specific membrane antigen (PSMA, > 250,000 receptors/cell on the LNCaP human prostate cancer cell line)25, the somatostatin-2a receptor (Sst2a, >28,000 receptors/cell on the ZR-75-1 human breast cancer cell line)26 or various integrin receptors (e.g. integrin αvβ3, >120,000 receptors/cell on the U87MG human glioma cell line)27.