Though it was once thought that some animal species (e.g., rodents) were insensitive to OPIDP, only mice appear to be somewhat resistant [18]. Age, on the other hand, is an important determinant of susceptibility, with young animals displaying more resistance (e.g., in young chicks the threshold for NTE inhibition and aging is > 90% vs. 70% in adult hens; [19]). Hypotheses to explain the consequences of these OP-NTE interactions include a loss of non-esterase functions of NTE essential for the axon, or a gain of toxic function of phosphorylated/aged NTE [13]. Though reductions in axonal transport have been found to precede overt clinical signs, the exact chain of events occurring between phosphorylation and aging of NTE and axonal degeneration remain obscure. Similarly, the physiological function(s) of NTE are not yet known, though recent studies (see Section 3.5) have attempted to clarify it.