Lower panel: Genetic variants that regulate the expression of other genes are known as expression quantitative trait loci (eQTL) and they are typically found close to the transcription start site (arrow) of the gene that they regulate (blue rectangle). However, they also occur in exons and may thus affect the structure and stability of messenger RNA transcripts (Montgomery et al., 2010; Pickrell et al., 2010). Upper panel: Left, although the majority of genes do not show altered expression in response to vaccination, a minority do show different levels of expression before and afterwards. Centre, these changes in expression are influenced by the individual’s genotype at other loci. Right, for some genes, changes in expression correlate with the individual’s antibody response to vaccination. These genes could be used as biomarkers to predict whether an individual will respond successfully to a vaccine.