As a result of its good sensitivity, ApEn has also been used to process many different physiological signals in studies related to major depressive disorder, including irregular secretion of cortisol and adrenocorticotropin [58,59,60,61], diurnal elevations in plasma interleukin-6 levels [62], gastric dysrhythmia [63], cardiorespiratory coupling [64], heart rate variability [65,66], and EEG signals [22]. The gene discussed is IL6; the disease is major depressive disorder.