Tyrosinase (TYR) and Tyrosinase-related protein 1 (TYRP1) processing and the melanin biosynthetic pathway in the melanocyte and in the melanosome, respectively. A) Schematic representation of melanosome biogenesis in the melanocyte and trafficking of TYR and TYRP1 from the endoplasmatic reticulum (ER) via Golgi apparatus to the developing melanosome. Places are indicated where the transport or sorting of TYR and TYRP1 from the synthesis in the ER to the melanosomes is abolished caused by mutations in the four genes found to be responsible for OCA (OCA1 to OCA4, respectively) (adapted from ). B) Illustration of the melanin (eumelanin/pheomelanin) biosynthesis pathway in the melanosome. DHI: 5,6-Dihydroxyindole, DHICA: 5,6-Dihydroxyindole-2-carboxylic acid, TYR: tyrosinase, TYRP1: Tyrosinase-related protein 1 (DHICA oxidase), DCT: Dopachrome tautomerase.