Epigenetics literally means "on top of" the genetic code and refers to modifications to DNA or chromosomes that alter gene expression. Epigenetic mechanisms act at the interface of genetic and environmental risk factors involved in complex human disorders such as autism. The first layer of epigenetic code is the addition of methyl groups to specific pairs of nucleotides (CpG sites), called DNA methylation. Recent advances in sequencing technologies have enabled the first glimpses of the genome-wide methylation patterns or "methylome" of humans. These preliminary methylome studies have revealed important differences in the methylome landscape between different human tissues. To an epigeneticist, these feel like seeing the first satellite images of the earth's landscape from space. In a serendipitous observation of a comparison of methylome sequences between a human neuronal cell line and a human skin cell line, we discovered that particular features of the methylome landscape called partially methylated domains (PMDs) could identify a subset of genes with specific expression in neurons. Remarkably, the genes in these domains were significantly enriched for genes already identified as autism candidate genes. In addition, the genes identified by tissue-specific methylome data were significantly enriched for functions in neuronal development and synapse function. Autism spectrum disorder (ASD) includes a complex mixture of human disorders with equally complex etiologies. Although research into genetic causes of autism has been extensive, autism cases with testable genetic mutations remain at around 10%-15% of autism cases. Autism is currently diagnosed by a behavioral checklist and treated by intensive behavioral interventions. There is currently no molecular test for autism risk in expectant mothers or newborns, no molecularly based pharmacological therapies, and no real understanding of the environmental risk factors that could aid in prevention or improved treatments of ASD. Unlike genetic testing that can be performed on easily accessible cells in the body like blood or saliva, epigenetic marks such as DNA methylation vary by tissue type. The most relevant tissue for understanding epigenetics of ASDs would be fetal brain, but this is obviously not accessible from live births. However, human term placenta is a unique temporary human tissue that is easily accessible at birth and can provide an important epigenetic "window" into fetal life and environmental risk factors that may have predisposed to epigenetic alterations and autism risk. This proposal is based on preliminary data from human placenta showing that many autism candidate genes are within placental-detected methylation domains by methylation sequencing. The main objective of this study is to identify a panel of epigenetic "biomarkers" in placenta that are predictive of ASD. To accomplish this goal, we plan to perform methylation sequencing and domain analyses on 60 human placenta samples obtained from the MARBLES (Markers of Autism Risk in Babies -- Learning Early Signs) Study, at the UC Davis M.I.N.D. Institute. This is a prospective study uniquely designed for genetic and environmental risk and protective factors in ASD risk. At the end of this study, we expect to have a validated panel of epigenetic biomarkers that could be used on human placental DNA in order to predict future diagnosis of ASD and to guide future treatment and prevention to improve the lives of children and families affected by ASD.