PDCD1 and melanoma: The biochemical processes and molecular targets that are currently under investigation for melanoma imaging are associated with the immune response (e.g., PD-1/PD-L1 [266], indoleamine 2,3-dioxygenase (IDO) [267]), TME dynamics (e.g., fibroblast activation protein (FAP) [268]), melanomagenesis (e.g., melanin [269]), invasion (e.g., melanocortin receptor subtype 1 (MC1R) [270]), or angiogenesis (e.g., integrins [271]) and hence many other novel antibodies and angiogenesis-related molecules continue to reside in the arena of research and development.