Many individual genetic variants contribute to ASD, but newer evidence emphasizes how these genes converge on a common set of pathways controlling the growth, development, shape and function of brain cells. In some people with autism, head size is enlarged starting very young, suggesting a type of autism that has specific genetic features. A biomarker that reflects fundamental differences in brain growth might be useful for early detection of ASD. This study will focus on small fragments of RNA called microRNA (miRNA) that control protein synthesis. Specifically, dysregulation of some miRNAs may lead to too much or too little protein in certain cells. The levels of these miRNAs can be measured in blood, which makes it a potential biomarker, and this project will reveal how specific miRNA pathways contribute to head growth in ASD.