Proteins in the PPI network also included the dopaminergic system (such as DRD1, SLC6A2, SLC6A3, SLC6A4, MAOA, and MAOB) and serotonergic system (such as HTR2A and HTR3A), whose mechanisms of action are closely related to the transmission of monoamine neurotransmitters, most of which are reportedly associated with insomnia. This evidence concerns the gene SLC6A4 and insomnia.