Compared to AOM-treated WT mice, M1R-deficient mice had slightly increased expression of Pdgf (2.0 ± 0.11 vs. 2.46 ± 0.16 fold respectively, p < 0.05) but significantly decreased expression of Pdgf-R (3.05 ± 0.84 vs. 1.17 ± 0.28 fold respectively, p < 0.05; Fig. 2E), suggesting that Chrm1 deficiency reduces activated HSC proliferation in part through down-regulation of PDGF-R. This evidence concerns the gene PDGFRB and infectious otitis media.