The invention provides a method of identifying compounds that either increase or decrease skin and/or hair pigmentation, the method comprising determining the ability of a test compound to modulate NCKX-mediated calcium ion movement across a membrane (e.g. NCKX5). The method may comprise the steps of exposing a membrane comprising a NCKX molecule or variant, fusion or derivative thereof to a test compound and measuring either directly or indirectly the calcium ion concentration on one or both sides of the membrane. The invention also relates to kits, nucleic acid molecules, polypeptides and cells useful in the method of the invention.