Variation in the human genome has long been considered to contribute to individual differences in disease susceptibility and drug response. But a key question for clinical practice is whether knowledge of a patient’s genotype could be useful for stratifying disease risk or guiding treatment. In the linked systematic review (doi:10.1136/bmj.d4588) Bauer and colleagues report a systematic review and meta-analysis of studies examining the association of variation in the CYP2C19 gene and atherothrombotic events during treatment with clopidogrel.1

The sequence of the human genome is now known,2 as are the positions of the several million nucleotides that differ most commonly from one person to the next and their inheritance patterns in different human populations. Laboratory and analytical techniques now permit rapid cost effective direct (and indirect) genotyping of many single nucleotide polymorphisms (SNPs) in the genomes of many thousands of people to gain insight into the regions that influence disease related biomarkers, susceptibility to common diseases, or the response to widely prescribed drugs.

By 2011, nearly 1000 such genome-wide association studies had reported their findings (figure⇓).3 Genome-wide association studies of disease risk are typically large and collaborative, and the results have usually been replicated in independent samples before publication. …