A reliable biomarker of HDL function is the HDL cholesterol efflux capacity (CEC), and this was shown for the first time to be reduced in people with OSA, where PBMC-derived macrophages from patients with OSA showed a suppressed ability to efflux cholesterol through the ATP-binding cassette transporter 1 (ABCA1) [206]. Here, ABCA1 is linked to obstructive sleep apnea syndrome.