Human association studies have centered on RNA obtained from leukocytes, predominantly Epstein-Barr virus (EBV) transformed B cell lines from HapMap individuals [3] or unrelated trios [1]. A large scale genetical genomics linkage study has recently identified more than 1000 cis regulatory loci across the genome, in primary cryopreserved human leukocytes [2], although these are broad genomic regions rather than variants in more precisely defined LD blocks. However, the former studies are limited by their ability to assess transcripts that are significantly expressed at the point of RNA isolation. Cell extraction methods, cryopreservation and EBV transformation all affect individual mRNA expression levels. These variations in RNA analysis make the choice of analysis tissue of paramount importance.
