A rare variant of human IL-33 (NM_001199640:exon7:c.487-1G>C (rs146597587-C) is associated with lower eosinophil counts and a reduced risk of asthma in Europeans; this is because rs146597587-C results in a premature stop codon that leads to the expression of truncated IL-33 (deletion of C-terminal residues 205–270), which has normal intracellular localization but cannot bind IL-33R/ST2 to activate ST2-mediated intracellular signaling [43,44]. Here, IL1RL1 is linked to asthma.