IL13 and asthma: In the first genome-wide association study in severe or difficult to treat asthma,25 which used the Epidemiology and Natural History of Asthma: Outcomes and Treatment Regimes (TENOR) cohort, associations with known asthma loci were found—eg, with single nucleotide polymorphisms (SNPs) in the RAD50/IL13 and HLA-DR/HLA-DQ regions.