Chitinase-3-like protein (YKL-40) is a glycoprotein thought to be involved in extracellular matrix remodelling and inflammation.92–94 YKL-40 CSF levels are increased in monogenic and sporadic cases of frontotemporal dementia and amyotrophic lateral sclerosis compared to both asymptomatic mutation carriers and controls.95 These findings have been pathologically validated with elevated YKL-40 prevalent in the amyotrophic lateral sclerosis motor cortex, frontal cortex and spinal cord.96,97 YKL-40 CSF levels are increased in frontotemporal dementia with TDP inclusions compared to controls. Here, CHI3L1 is linked to amyotrophic lateral sclerosis.