Coding mutations are linked to neurodevelopmental disorders (NDDs), such as autism, but predicting the effects of non-coding mutations is difficult. Researchers assessed 997 mutations from NDD families using various assays. A massively parallel reporter assay (MPRA) with polysomes from cell lines identified over 100 mutations that changed translation, with some also affecting protein production in patient lymphoblastoid cell lines. Since the function of untranslated regions (UTRs) can differ by cell type, the study optimized Cre-dependent MPRAs for evaluation in neurons in vivo. Findings revealed that neurons have distinct regulatory principles influenced by 5’ UTRs and identified mutations that affect translational activity. The team also examined whether polysome-MPRAs could predict changes in protein production linked to canonical open reading frames (ORFs). Correlation was reasonable only for mutations that altered UTR structure. This research benchmarks multiple methods for evaluating the effects of 5’ UTR mutations and highlights functional mutations in known NDD genes, including LRRC4 and ZNF644.