<qa>  What is the primary aim of this study?The primary aims of this study are to evaluate the efficacy and tolerability of high-dose folinic acid for improving the closely associated symptoms of language impairment in children with autism spectrum disorder ASD. Improvement in delayed language may also benefit the core ASD problem of social communication. The study will also focus on identification of biomarkers in pre-specified subgroups of children with ASD that may moderate positive response to folinic acid. The study model is that high-dose folinic acid will improve language and set the stage for improved social communication in children with ASD and moderate language impairment. To test whether folinic acid is superior to placebo, 160 children age 5 to 12 yrs 6 months, inclusive with ASD and moderate language will be randomly assigned to folinic acid or placebo for 12 weeks under double-blind conditions. The study team will also test whether abnormalities in folate-dependent pathways, such as dysfunctional transport of folate across the blood-brain barrier, will moderate positive response to folinic acid treatment. 