{ diff(x1, t) = (1*k12*k2/(1 + (k6*(1 + (x3/k7)^k9))^k8) + (-1)*k12*k3*x1)/k12,
  diff(x2, t) = (1*k12*x1 + (-1)*k12*k4*x2)/k12,
  diff(x3, t) = (1*k12*k1*x2 + (-1)*k12*k5*x3)/k12 }