{ diff(x1, t) = 0,
  diff(x2, t) = (1*k20*k17*k15/k16*(k21/k6)^k2/(1 + (k21/k6)^k2) + (-1)*k20*k18*k15/k16*(x3/k7)^(k12 - (k12 - k13)*x4/k8/(1 + x4/k8))/((x3/k7)^(k12 - (k12 - k13)*x4/k8/(1 + x4/k8)) + (1 + (x4/k9)^k11)/(1 + k10^(k12 - (k12 - k13)*x4/k8/(1 + x4/k8))*(x4/k9)^k11)))/k20,
  diff(x4, t) = (1*k20*k18*k15/k16*(x3/k7)^(k12 - (k12 - k13)*x4/k8/(1 + x4/k8))/((x3/k7)^(k12 - (k12 - k13)*x4/k8/(1 + x4/k8)) + (1 + (x4/k9)^k11)/(1 + k10^(k12 - (k12 - k13)*x4/k8/(1 + x4/k8))*(x4/k9)^k11)) + (-1)*k20*k19*k15/k16*x4/k8/(x4/k8 + 1))/k20,
  diff(x5, t) = 0 }