In cases with assumed diagnosis of frontotemporal lobar degeneration (FTLD) , literatures around early 20^th century had already described the presence of intra neuronal structures with characteristic morphology. However, it has long been difficult to making sense of the neuropathological findings and corresponding clinical phenotypes. Recent refinement of the disease concept and remarkable progresses in molecular genetics and biochemistry revealed multiple genetic causes and the identity of several accumulating proteins. These advances allowed further classification and understandings of clinico‐neuropathological relationship in FTLD. This review describes molecular basis of FTLD and briefly introduce our study regarding FTLD due to C9orf72 repeat expansion. No potential conflicts of interest were disclosed.