DNA methylation (mDNA) plays an important role in the pathogenesis of neurodevelopmental disorders (NDDs), however its use in diagnostic testing has been largely restricted to a handful of methods for locus-specific analysis in monogenic syndromes. Recent studies employing genome-wide methylation analysis (GWMA) have explored utility of a single array-based test to detect methylation changes in probands negative by exome sequencing, and to diagnose different monogenic NDDs with defined epigenetic signatures. While this may be a more efficient approach, several significant barriers remain. These include non-uniform and low coverage of regulatory regions that may have CG-rich sequences, and lower analytical sensitivity as compared with locus-specific analyses that may result in methylation mosaicism not being detected. A major challenge associated with the above technologies, regardless of whether the analysis is locus specific or genome wide, is the technical bias introduced by indirect analysis of methylation. This review summarizes evidence from the most recent studies in this field and discusses future directions, including direct analysis of methylation using long-read technologies and detection of 5-methylcytosine (5-mC or total mDNA) and 5-hydroxymethylacytosine (5-hmC) as biomarkers of NDDs.

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