Pathological developments leading to amyotrophic lateral sclerosis (ALS) and 
frontotemporal lobar degeneration (FTLD) are associated with misbehavior of 
several key proteins, such as SOD1 (superoxide dismutase 1), TARDBP/TDP-43, FUS, 
C9orf72, and dipeptide repeat proteins generated as a result of the translation 
of the intronic hexanucleotide expansions in the C9orf72 gene, PFN1 (profilin 
1), GLE1 (GLE1, RNA export mediator), PURA (purine rich element binding protein 
A), FLCN (folliculin), RBM45 (RNA binding motif protein 45), SS18L1/CREST, 
HNRNPA1 (heterogeneous nuclear ribonucleoprotein A1), HNRNPA2B1 (heterogeneous 
nuclear ribonucleoprotein A2/B1), ATXN2 (ataxin 2), MAPT (microtubule associated 
protein tau), and TIA1 (TIA1 cytotoxic granule associated RNA binding protein). 
Although these proteins are structurally and functionally different and have 
rather different pathological functions, they all possess some levels of 
intrinsic disorder and are either directly engaged in or are at least related to 
the physiological liquid-liquid phase transitions (LLPTs) leading to the 
formation of various proteinaceous membrane-less organelles (PMLOs), both normal 
and pathological. This review describes the normal and pathological functions of 
these ALS- and FTLD-related proteins, describes their major structural 
properties, glances at their intrinsic disorder status, and analyzes the 
involvement of these proteins in the formation of normal and pathological PMLOs, 
with the ultimate goal of better understanding the roles of LLPTs and intrinsic 
disorder in the "Dr. Jekyll-Mr. Hyde" behavior of those proteins.