CX3CR1 and amyotrophic lateral sclerosis: Given the dynamic changes of the CX3CL1/CX3CR1 axis and the unbalanced M1/M2 microglia activation in the pathological process of ALS, elucidation of the molecular mechanism through which the CX3CL1/CX3CR1 axis regulates microglial activation may render the CX3CL1/CX3CR1 axis as a therapeutic target for ALS.