A study in a mouse model of bleomycin-induced fibrosis showed the involvement of serotonin and its receptors in pulmonary fibrosis, with increased levels of serotonin being found in the lung and increased levels of serotonin receptors 5-HT2A and 5-HT2B being found in the fibrotic lungs [16]. This evidence concerns the gene HTR2A and fibrosis.