CHI3L1 and Alzheimer disease: While YKL-40 and GFAP are well-known markers of astrogliosis, being upregulated in reactive astrocytes [13–21, 23, 24], CHIT1 is a microglia/macrophage protein that cleaves N-acetyl glucosamine polymers (mainly found in chitin) in AD amyloid plaques [25] and is highly expressed in ALS spinal cord in association with microglial activation [22, 23].