TREM2 and Alzheimer disease: Furthermore, as the ε4 allele of apolipoprotein E—a TREM2 ligand—is the dominant genetic risk factor for late‐onset AD (LOAD) [4], and TYROBP—a TREM2 adaptor protein—has been identified as the key regulator of LOAD using an integrative network‐based approach [5], TREM2 has emerged as an important signaling molecule in AD.