{ diff(x1, t) = (1*k1*k2*x2 + (-1)*k3*x1/(k4 + x1))/k1,
  diff(x2, t) = (1*k5*(1 - x2)/(k6 + 1 - x2) + (-1)*k7*x3*x2/(k8 + x2))/k1,
  diff(x3, t) = (1*x1*k9*(1 - x3)/(k10 + 1 - x3) + (-1)*k11*x3/(k12 + x3))/k1 }