ADA and amyotrophic lateral sclerosis: Evidence for increased adenosine levels in the cerebrospinal fluid of patients with ALS (n = 12) [61] was already available in the late 1990s, but it has only recently been demonstrated that the extracellular adenosine concentration, which can be increased by loss of astrocyte adenosine deaminase (ADA), is critical to induce motor neuron toxicity in ALS [62].