Figure 1a shows a P-value Manhattan plot for twin-based h2 for 18,392 genes (selecting for each gene the transcript with the largest h2), based on twin zygosity comparisons (inset). The h2 had mean±SD of 0.101±0.142 (0.138±0.153 for expressed genes), with maximum estimated h2=0.905. We conservatively highlight 777 genes with significant heritability (q < 0.05, 4.2% of the genes on the microarray), applying k-means clustering and analysis of genomic location. The 777 genes yielded 9 expression clusters (Figure 1b, Supplementary Table 1). Mean within-cluster r ranged from 0.46 to 0.006. Cluster identity was supported by significantly higher connectivity in protein-protein interaction databases34 and GO pathways35 (Supplementary Table 1). Numerous clustered genes displayed expression patterns similar to other tissues, including brain,35 suggesting broader tissue relevance. Regional clustering indicated enrichment for immune function (Supplementary Table 2, e.g., IgG Fc fragment receptors at chr1:161–162 mb and the MHC region at chr6:31–33 mb), while other regions showed fewer heritable genes (e.g., neuronal protocadherin gene cluster at chr5:140–141 mb and epidermal keratin gene clusters on chr17:39–40 mb and chr21:31–32 mb). Figure 1c shows that heritability is
