Gene Ontology (GO) is the knowledge base that describes the functions of genes in terms of biological processes, molecular functions, and cellular components [44]. Genes associated with the GO categories were used as the prior gene sets for the enrichment analysis. The GO database was obtained using the R package org.Hs.eg.db (version 3.13). In addition, annotations for developmental stages of brain regions were used to explore the specificity of PC1-related genes to developmental stages (http://genetics.wustl.edu/jdlab/csea-tool-2). Briefly, RNA-seq data were obtained from BrainSpan (http://www.brainspan.org/), an atlas of the developing human brain, with samples aggregated into six major regions across 10 developmental periods (i.e., a total of 60 types). Then a specificity index probability (pSI) was calculated for genes of the samples to assess if they were more expressed in a specific sample relative to others, with a lower pSI meaning more stringent specificity [45, 46]. Genes associated with the stages of cortical development (pSI < 0.01) were used as the prior gene sets for the enrichment analysis. As the Combined-PC1 was derived from cortical thickness, we only focused on the gene
