What causes autism is a mystery. One ＿＿（36）＿＿ is that a phenomenon called the cellular-danger response (CDR) lies at the root of it. The CDR makes cells put their ordinary activities ＿＿（37）＿＿ and instead switch on their defence systems, in reaction to high levels in the bloodstream of chemicals called purines. These are important and widespread ＿＿（38）＿＿ : ATP, a molecule that shuttles energy around cells, is a purine; so are half the "genetic letters" in DNA. Cells under viral attack tend to shed them. Too many of them in the blood can thus be a signal of viral infection. In that case ＿＿（39）＿＿ the CDR makes perfect sense. But studies have shown that people with autism (and also those with some other brain conditions, such as schizophrenia) often seem to have chronic CDR. The purine signal has somehow ＿＿（40）＿＿ in the "on" position. Yet why this happens is obscure.