{ diff(x1, t) = (1*k21*k1*x1 + (-1)*k21*k1*x1^2/k20 + (-1)*k21*k3*x2*x1/(k4 + x1) + 1*k21*k8*x4*x1/(k9 + x4))/k21,
  diff(x2, t) = ((-1)*k21*k14*x2 + 1*k21*k2*x1/(1 + k13*x4) + 1*k21*k6*x2*x3/(k7 + x3)*(k6 - k17*x4/(k18 + x4)))/k21,
  diff(x3, t) = ((-1)*k21*k5*x3 + 1*k21*k15*x2*x1/((k12 + x1)*(1 + k11*x4)))/k21,
  diff(x4, t) = (1*k21*k16*x1^2/(k19^2 + x1^2) + (-1)*k21*k10*x4)/k21 }