It is sometimes inconvenient to use negatively marked clones, especially when generating very small patches of cells, where it is more difficult to see a dark spot on a bright background than a bright spot on a dark background. It is possible to create positively marked clones using the so-called MARCM ("Mosaic Analysis with a Repressible Cell Marker", pronounced [mark-em]) system, developed by Liqun Luo , a professor at Stanford University , and his post-doc Tzumin Lee who now leads a group at Janelia Farm Research Campus . This system builds on the GAL4/UAS system, which is used to express GFP in specific cells. However a globally expressed GAL80 gene is used to repress the action of GAL4, preventing the expression of GFP. Instead of using GFP to mark the wild-type chromosome as above, GAL80 serves this purpose, so that when it is removed by mitotic recombination , GAL4 is allowed to function, and GFP turns on. This results in the cells of interest being marked brightly in a dark background.