TPH2, Mash1, and 5-HT were mainly expressed in neurons but rarely expressed in astrocytes or microglia in the piriform cortex (Pir), while they were not overlapping in the other cerebral regions, implying that the neurons in Pir might be involved in the role of 5-HT in ALS. The gene discussed is PIR; the disease is amyotrophic lateral sclerosis.