Unlike most other mutagens, the molecular lesions caused by EMS are essentially random, ensuring that most genes of interest will be targeted and that multiple lesions will be found in each gene. Whole-genome sequencing now allow us to reliably and efficiently map EMS induced lesions at very reasonable costs ( Blumenstiel et al., 2009 and Wang et al., 2010) (H.J.B., unpublished

data). The identification of novel genes that affect specific biological processes in a specific tissue are based on creating mosaic animals (Xu and Rubin, 1993). Flp-mediated mitotic recombination screens check details result in the generation of homozygous mutant tissue in an otherwise heterozygous animal, limiting the effect of a possible detrimental or lethal mutant phenotype at an earlier developmental stage. Advantageously, such screens can often be designed as F1 screens where single progeny of mutagenized flies can be directly screened, mutations isolated, and balanced to generate stable stocks if the screen does not jeopardize viability and fertility of the heterozygous animals that carry clones. These screens are most conveniently performed with

EMS. Forward genetic Flp/FRT screens are based on creating clones in specific cells, tissue or organs using specific click here Flp drivers ( Figure 6). Flp expression results in homozygous mutant tissue associated with a phenotypic outcome that can be scored easily. The most widely used driver is an eye specific driver, ey-Flp ( Newsome et al., 2000), or ey-GAL4; UAS-Flp ( Stowers and Schwarz, 1999). To obtain clones that are large enough it is important to use a driver that is expressed early in development. Moreover,

clone size can be enhanced with the use of homologous chromosomes that carry a recessive cell lethal mutation, or a Minute. The large clones in the eye have allowed screening for morphological defects of eye cells Tolmetin ( Newsome et al., 2000), simple behavioral paradigms such as phototaxis ( Verstreken et al., 2003), electrophysiological function using electroretinograms ( Ohyama et al., 2007), or bristle abnormalities on the head cuticle ( Tien et al., 2008). These screens can be also combined with different MARCM strategies (see above). Forward genetic screens generally require a strategy to genetically and/or molecularly map the mutation. In the case of transposons, the insertion site is often known or can be easily mapped (Hui et al., 1998 and Bellen et al., 2011). Mutation mapping becomes more challenging for EMS mutagenesis.