Brain functions of PCSK9 were recently evaluated in animal studies by creating a knock-out of the culprit protein in mice. A downregulation of PCSK9 expression led to impaired synaptic plasticity of hippocampal dendritic spines, thus affecting cognitive function and performance [8]. Interestingly, patients with MDD display a similar pattern of hippocampal synaptic dysfunction [14]. The gene discussed is PCSK9; the disease is major depressive disorder.