In MS, several processes are suspected to lead to low serotonin levels, including the following: (1) the differential availability of the 5-HT transporter in the CNS, (2) the reduction in total tryptophan levels, and (3) the diversion of the amino acid from its synthetic pathway (San Hernandez et al. 2020). The gene discussed is SLC6A4; the disease is myeloid sarcoma.