CHI3L1 and asthma: YKL-40 is encoded by the chitinase 3-like1 gene, CHI3L1, and several studies of single nucleotide polymorphisms (SNPs) of CHI3L1 show that genetic variations of CHI3L1 have an impact on inter-individual serum YKL-40 levels and asthma susceptibility [11]–[14].