COL9A2 and hearing loss disorder: In terms of gene expression patterns, these four cell clusters are highly expressed in Col2a1, Col9a1, Col9a2, and Col11a2, which are the major gene members of the Col family, and mutations in these genes are associated with hearing loss (Richards et al., 2013; Hofrichter et al., 2019; Kjellström et al., 2021).