Dysregulation of histone methylation has emerged as a major driver of neurodevelopmental disorders including intellectual disabilities and autism spectrum disorders. Histone methyl writer and eraser enzymes generally act within multisubunit complexes rather than in isolation. However, it remains largely elusive how such complexes cooperate to achieve the precise spatiotemporal gene expression in the developing brain. Histone H3K4 methylation (H3K4me) is a chromatin signature associated with active gene-regulatory elements. We review a body of literature that supports a model in which the RAI1-containing H3K4me writer complex counterbalances the LSD1-containing H3K4me eraser complex to ensure normal brain development. This model predicts H3K4me as the nexus of previously unrelated neurodevelopmental disorders.