Green fluorescent tortoiseshell mice

X-inactivation is the mechanism by which mammals achieve X-linked gene dosage equivalence between the sexes. This event occurs during the early stages of postimplantation embryonic development when XX females silence one of their two active X chromosomes. Somatic X-inactivation is a random event, and so females are in fact mosaic, comprising two populations of cells, each exhibiting exclusive expression of genes located on one or other of the two X chromosomes. The choice of which X chromosome remains active is stably maintained and clonally inherited. These six, two day old mouse pups are siblings, with all but one (third from the left) having inherited a ubiquitously expressed X-linked GFP transgene, X GFP , from their X GFP X mother (see Genesis 2001, 2 29 9:133-140). The activity of the transgenic chromosome in skin cells can be assessed under fluorescent illumination. Green fluorescence indicates expression of the transgenic X GFP chromosome, and lack of fluorescence indicates expression of a non-transgenic X chromosome. The two mice whose skin is uniformly green fluorescent (far left and second from right) are X GFP Y males, whereas the three mice exhibiting varying degrees of a tortoiseshell type of green fluorescent pattern (second from left, third from right and far right) are X GFP X females. The X GFP X females have undergone X-inactivation and so have mosaic green fluorescence in their skin; this is the same phenomenon that is seen in the fur of tortoiseshell cats.