These pathways co-exist, but competition between these two enzymes favors arginase in humans (Wu et al., 2009; Yi et al., 2009; Guo et al., 2012), and iNOS activity is decreased in human AD, resulting in upregulation of the arginase pathway (Colton et al., 2006; Yi et al., 2009; Liu et al., 2014). Here, NOS2 is linked to Alzheimer disease.