We first report the heritability of steady-state transcription in peripheral blood for 43,628 transcripts from 18,392 genes from 2,752 individual twins in the Netherlands Twin Registry (NTR, Table 1). The U219 platform includes alternate 3′ sequences of well-annotated genes, and we refer to each of the 43K probe sets as a “transcript” (1–18 transcripts per gene, mean 2.4). Careful annotation was performed for the platform, which compares favorably to RNA-Seq (Supplementary Note).33 Subjects were from 1,444 twin pairs (both members of 1,308 pairs, 95.1% of subjects, and one member from 136 pairs). The 1,308 complete pairs consisted of 690 MZ pairs (52.8%, 209 male and 481 female MZ pairs) and 618 DZ pairs (47.2%, 110 male, 256 female, and 252 opposite-sex DZ pairs). Expression QC included zygosity/sex confirmation, randomization for sex and zygosity balance, sample identity checks, and dropping low-quality samples. Primary analyses are based on Robust Multi-array Average (RMA) expression estimates, filtered to exclude probes containing SNPs or mapping non-uniquely, with each transcript transformed to an exact normal distribution for robust analysis.
