Several studies show decreased leptin levels in CSF and plasma in AD patients (Holden et al., 2009; Lieb et al., 2009; Bigalke et al., 2011; Chakrabarti, 2014; Johnston et al., 2014; Khemka et al., 2014b; Baranowska-Bik et al., 2015) whereas increased leptin levels (Bonda et al., 2014; Yin et al., 2018) and unaffected levels (Maioli et al., 2015; Oania and McEvoy, 2015) in CSF and cerebral tissue were also reported. The gene discussed is LEP; the disease is Alzheimer disease.