{ diff(x1, t) = (1*k25*k15*k16*(1 - k17/k18) + (-1)*k25*k2*x1 + (-1)*k25*k3*x1*x3)/k25,
  diff(x2, t) = (1*k25*(k4 + k5*x4^k6/(k7^k6 + x4^k6)) + (-1)*k25*k8*x2)/k25,
  diff(x3, t) = (1*k25*k9*x2 + (-1)*k25*k10*x3)/k25,
  diff(x4, t) = (1*k25*k11*x5*k12^k13/(k12^k13 + x1^k13) + (-1)*k25*k14*x4)/k25 }