Abnormal protein citrullination (PC) and dysregulation of peptidyl arginine deiminases (PADs) are associated with numerous pathological conditions, including inflammatory diseases and neurodegeneration. Inhibition of PADs in animal disease models has shown therapeutic efficacy, suggesting a role of PC in pathogenesis. To determine whether PC contribute to amyotrophic lateral sclerosis (ALS), a deadly neurodegenerative disease characterized by loss of motor neurons, paralysis, and eventual death, we investigated PC in two transgenic mouse models of ALS. Both models show that PC and PAD2 increase progressively in reactive astrocytes, while decreasing in neurons. Importantly, PC accumulates progressively in protein aggregates that contain the myelin proteins PLP and MBP during disease progression. Finally, increased PC and PAD2 expression spatially correlate with areas of the CNS with the most severe motor neuron degeneration. These findings are replicated in human ALS tissues. Ongoing and future studies are focused on the impact of PAD2 knockout in ALS mouse models and mass spectrometry analysis of citrullinated proteins. Our results suggest that altered PC is an integral part of the neurodegenerative process and potential biomarkers for disease progression in ALS. Moreover, increased PC may contribute to disease-associated processes such as myelin protein aggregation, myelin degeneration, and astrogliosis.