In-depth study of the biology of any cell lineage is best performed while the cells reside in their natural morphological and physiological microenvironment. This task has been difficult to achieve for melanocytes because they make up only about one percent of the cellular milieu of the mammalian skin. The most effective strategy to ‘label’specific cell types is to express ‘molecular beacons’ under the control of cell lineage-specific gene promoters in transgenic mice. One such transgenic mouse model is Dct-LacZ, which expresses the betagalactosidase gene under the control of the melanocytespecific promoter of the dopachrome tautomerase (Dct) gene (Mackenzie et al., 1997). Beta-galactosidase cleaves its chromogenic substrate, X-gal,‘labeling’the transgene-expressing cells with blue color in visible light, which allows individual melanocytes to be visualized within the skin. The Dct-LacZ transgenic mouse model has been instrumental in studying the biology of melanocytes; for example, in helping define the existence and location of melanocyte stem cells. However, there are certain desirable features that this model lacks:(i) conditionally inducible labeling and,(ii) fluorescent label, so that it can be used to isolate melanocytes by fluorescenceactivated cell sorting (FACS) following in vivo manipulation.

The year 2010 saw publication of three mouse models that expressed fluorescent labels in melanocyte-specific manner. Mort et al.(2010) reported mice that express yellow fluorescent protein (YFP) in conjunction with Tyrosinase-driven Cre recombinase. Monahan et al.(2010) published a similar strategy to express green fluorescent protein (GFP) in melanocytes. However, in both of these models, fluorescence is conditional but not inducible.