This chapter describes how leucine-rich repeat transmembrane neuronal 1 (LRRTM1)'s involvement in handedness was discovered, and also the latest knowledge of its functions in brain development and disease. In searching for genetic effects on handedness and schizophrenia, it is therefore reasonable to approach complex and heterogeneous traits as multi-factorial, and to apply relatively model-free methods developed for human complex-trait genetics. Maximum likelihood variance-components modeling, with independent paternal and maternal quantitative-trait-locus (QTL) effects, also yielded an estimate of the paternal effect roughly five times that of the maternal effect at the peak of linkage. The single nucleotide polymorphisms (SNPs) were targeted within four positional and functional candidate genes on 2p12-p11 (LRRTM4, CTNNA2, LRRTM1, DNAH6). The chapter analyzes the SNPs rs1007371 and rs723524, which together can be used to construct a close proxy to the risk haplotype. The imprinting of LRRTM1 appears to be human or primate specific based on preliminary data.