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