Candidate gene studies of MD have generated many publications but few robust findings. At the time of writing (2013), searching for articles dealing with genetic association and MD returned more than 1,500 hits. Almost 200 genes have been subject to testing, many by multiple groups (Bosker et al., 2011, López-León et al., 2008). The difficulty, common in this area of research, is that few groups agree with each other. Resolution of conflicting results is usually attempted through meta-analysis and Table 2 summarizes data for 26 genes analyzed by meta-analysis, of which seven yield a significant (p < 0.05) result: 5HTTP/SLC6A4, APOE, DRD4, GNB3, HTR1A, MTHFR, and SLC6A3.Table 2Candidate Gene Meta-analysesReferenceNumber of StudiesNumber of CasesNumber of Controlsp ValueOR95% CIVariantMAFPowerNumber for 80% Power5-HTR2AAnguelova et al., 200377689590.5970.960.84–1.11rs63110.446.2%55,781Jin et al., 2013111,4912,9370.12NANArs63110.44NANA5HT-6RFukuo et al., 201047012,4220.4060.940.80–1.08rs18050540.178.4%19,0215HTTP/SLC6A4Clarke et al., 2010396,83614,9030.0071.091.02–1.1644 bp ins/del0.4365.8%9,575Anguelova et al., 2003119412,1100.1981.080.96–1.2244 bp ins/del0.4312.3%11,958Anguelova et al., 2003105922,094>0.5NANAintron 2 VNTR0.35NANAFurlong et al., 199842757390.0491.21.00–1.4544 bp ins/del0.4317.3%2,112Lasky-Su et al., 2005141,9613,4020.281.050.96–1.1444 bp ins/del0.4310.5%32,911López-León et al., 2008223,7525,707<0.051.111.04–1.1944 bp ins/del0.4351.6%7,356López-León et al., 20088NANANS1.330.78–2.27intron 2 VNTR0.35NANAACEWu et al., 2012152,4797,744NS1.151.02–1.3Ins/del intron 160.4565.9%3,465López-León et al., 200645865,169>0.10.850.55–1.3Ins/del intron160.4533%1,992López-León et al., 20088NANANS1.080.97–1.2Ins/del intron160.45NANABDNFGyekis et al.,
