{ diff(x1, t) = (-1)*k72*k38*k55*x1*x2/(k54*k48 + k50*k47 + k49*k46)/k72,
  diff(x2, t) = (1*k72*k38*k55*x1*x2/(k54*k48 + k50*k47 + k49*k46) + (-1)*k72*(k54*(k53*k62 + k52*k59 + k51*k56) + k50*(k53*k61 + k52*k58) + k49*(k53*k60 + k52*k57))*x2)/k72,
  diff(x3, t) = (1*k72*(k54*(k53*k62 + k52*k59 + k51*k56) + k50*(k53*k61 + k52*k58) + k49*(k53*k60 + k52*k57))*x2 + (-1)*k72*(k63 + (1 - k63)*k71)*x3)/k72,
  diff(x4, t) = 1*k72*(k63 + (1 - k63)*k71)*x3/k72 }