TREM2 and Alzheimer disease: The presence of this variant also reduces the cleavage of full-length TREM2 to a soluble extracellular fragment and, in both TREM2 risk variant carriers and in people with AD, less soluble TREM2 is present in cerebrospinal fluid (Kleinberger et al., 2014) suggesting TREM2 dysfunction may be a common feature in AD and not just in those AD patients carrying a loss of function variant.