Patients with ALS have high CSF levels of neurofilament light chain (15–18) and chitinases, for example, chitotriosidase-1 (CHIT1), chitinase-3-like protein 1 (CHI3L1), and chitinase-3-like protein 2 (CHI3L2), which are cellular markers of macrophages and microglia (19–22). The gene discussed is CHI3L2; the disease is amyotrophic lateral sclerosis.