In the past two decades, genetic association studies on OSA have identified only a few genetic polymorphisms related to the apnea hypopnea index (AHI) independently, such as TNF-α-308G/A and serotonin 2A receptor -1438G/A, even though about 30% to 40% of the variability in OSA severity can be explained by familial factors [14,15,16,17]. Here, HTR2A is linked to obstructive sleep apnea syndrome.