YKL-40, is a plasma protein mainly produced by inflammatory cells and cancer cells during tissue remodelling processes. Consequently, plasma YKL-40 levels are elevated in patients with diseases characterized by inflammation and cancer. Despite its role in many serious diseases, the genetic background for plasma YKL-40 still has not been systematically catalogued. In this study, we aimed to identify all genetic variants in CHI3L1 influencing plasma YKL-40 levels. For this, we used extreme phenotype approach: From 8899 individuals from the general population with measured plasma YKL-40, we resequenced the gene in 10% of the individuals with extremely high or extremely low plasma YKL-40 levels. In total, we identified 59 genetic variants, of which 8 were associated with plasma YKL-40 levels (p<10^-8). This information might be valuable for future studies exploring the causal roles of YKL-40 for human health. (By Alisa D. Kjaergaard, http://jmg.bmj.com/content/early/2013/09/23/jmedgenet-2013-101908 )