{ diff(x1, t) = (1*k18*k14*x1*(1 - k5*x1) + (-1)*k18*(k10*x2*x1 + k3*(1 - exp(-x4))*x1))/k18,
  diff(x2, t) = (1*k18*k13*x2*(1 - k4*x2) + (-1)*k18*(k8*x3*x2 + k9*x2*x1 + k2*(1 - exp(-x4))*x2))/k18,
  diff(x3, t) = (1*k18*(k15 + k16*x3*x2/(k6 + x2)) + (-1)*k18*(k7*x3*x2 + k11*x3 + k1*(1 - exp(-x4))*x3))/k18,
  diff(x4, t) = (1*k18*k17 + (-1)*k18*k12*x4)/k18 }