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Melanocytes are very specialised cells that produce melanin, the pigment responsible for skin, hair and retina colour. They originate during embryogenesis from precursor cells called melanoblasts. These are round shaped and unpigmented cells that migrate from the neural crest to the dermis and differentiate in highly pigmented and dendritic melanocytes after they have reached the skin and hair follicles. Melanoma is a skin cancer that arises form the uncontrolled proliferation of melanocytes. Its incidence is rapidly increasing and it fails to respond to common treatment such as chemotherapy and radiotherapy. While initially it is very pigmented and confined to the upper layers of the dermis, it subsequently invades underneath tissues and forms metastasis that are frequently unpigmented. Indeed it has been suggested that melanoma progression involves a sort of dedifferentiation of the melanocyte into a transformed melanoma cell that morphologically is more similar to a melanoblast then to the melanocyte it originated from. In this thesis we describe the development, optimisation and execution of a functional screening on the mouse melanocyte cell line Melan-a for the identification of new genes involved in melanocyte differentiation and melanoma progression. To do this, a mouse cDNA library was constructed in the pHMII retroviral vector. The marker chosen for the identification of the positive clones was pigmentation and the screening was based on the selection of genes that provoked the depigmentation of melanocytes. The conditions of the screening were optimised using the Agouti and activated Ras genes in reconstruction experiments. The positive clones isolated from the screening were characterised for their ability to inhibit melanin production when overexpressed in melanocytes, as well as to confer growth advantages and transformed phenotype.