Table 1. Genetic Modifiers of MeCP2. Drosophila Gene, Flybase ID Mammalian Homolog(s) crooked legs (crol), FBgn0020309 RE1 silencing transcription factor (REST) Sin3a, FBgn0022764 Sin3a Smrter (Smr), FBgn0024308 nuclear receptor co-repressor (N-CoR) Additional sex combs (Asx) FBgn0000141 corto, FBgn0010313 Mastermind like 2 (Maml2) Additional Sex Combs like 1(Asxl1) osa, FBgn0003013 AT-rich interaction domain 1a & 1b (Arid1a, Arid1b) pebble (pbl), FBgn0003041 Epithelial cell transforming sequence 2(Ect2) Sex combs on midleg (Scm) FBgn0003334 Sex combs on midleg homolog 1(Scmh1) tricornered (trc), FBgn0003744 Nuclear Dbf-related 1 & 2 (Ndr1, Ndr2) trithorax (trx), FBgn0003862 Mixed lineage leukemia (Mll) Modifier Allele c04670 (LOF) e0407 (LOF) dQ4 (LOF) e04377 (LOF) e04389 (LOF) 1 (GOF) EY07384 (LOF) XF23(LOF) 07128b (LOF) c03244 (LOF) e02822 (LOF) 00090 (LOF) 308 (LOF) EY09619 (LOF) UAS-osa (OE) 2 (LOF) 3 (LOF) 5 (LOF) 09645 (LOF) UAS-pebble (OE) e01989 (LOF) D1 (LOF) ET50e (LOF) M36 (LOF) M56 (LOF) UAS-scm (OE) XF24 (LOF) UAS-trc LD (OE) UAS-trc wtn (OE) 1 (hypomorph) E2 (amorph) KG04195 (LOF) sup sup enh sup sup enh sup sup sup sup sup sup sup sup enh sup sup sup sup enh sup sup sup sup sup enh sup sup sup enh enh enh Genetic Modifiers of MeCP2 MeCP2 alleles Full length D166 R294X sup sup enh sup sup enh sup sup sup sup sup sup sup enh enh sup sup sup sup enh sup sup sup enh enh enh no mod no mod no mod no mod no mod no mod no mod sup sup sup sup enh sup sup sup sup sup sup sup sup enh sup sup sup enh sup sup sup sup sup sup sup no mod no mod no mod no mod no mod no mod LOF: loss of function, GOF: gain of function, OE: overexpression. no mod: no clear modification with this particular allele. doi:10.1371/journal.pgen.1000179.t001 performed and demonstrated that the modifiers did not alter the level of MeCP2 protein (Figure S2). Each modifier line found to alter the full-length MeCP2 phenotype was also investigated in the context of the D166 and R294X MeCP2 alleles to determine if the modification was dependent upon a specific MeCP2 domain. For the MeCP2 D166 allele, all genetic modifiers behaved similarly to the full-length MeCP2 allele (Table 1, Figure S3). Since the MeCP2 R294X allele does not dramatically alter the structure of the eye, suppression was assessed primarily by gain in the amount of eye pigmentation. Enhancement was assessed by increased loss of pigmentation and/or disruption in the external structure of the eye. We found similar phenotype modifications as with full-length MeCP2 with two interesting exceptions (Table 1 and Figure S4). Partial loss of Sin3A function, which enhances full-length MeCP2 (compare Figures 3B and 3C), suppresses MeCP2 R294X phenotypes (compare Figures S4D and S4A). Partial loss of trx function, which enhances the full-length MeCP2 phenotype (compare Figures 4B and 4I), but, in the case of the trxE2 allele suppresses the R294X phenotype (compare Figures S4A and S4P). The candidate suppressor genes were then further tested against the full-length MeCP2 allele in a second independent assay using the L3 wing vein phenotype (Figure 5A–E). Indeed, alleles of Asx, osa, Scm and trc are able to decrease the penetrance of the L3 wing vein phenotype. Furthermore, loss of function of osa and PLoS Genetics | www.plosgenetics.org 5 September 2008 | Volume 4 | Issue 9 | e1000179