Xenopus oocytes convert a continuously variable stimulus, the concentr
ation of the maturation-inducing hormone progesterone, into an all-or-
none biological response-oocyte maturation, Here evidence is presented
that the all-or-none character of the response is generated by the mi
togen-activated protein kinase (MAPK) cascade. Analysis of individual
oocytes showed that the response of MAPK to progesterone or Mos was eq
uivalent to that of a cooperative enzyme with a Hill coefficient of at
least 35, more than 10 times the Hill coefficient for the binding of
oxygen to hemoglobin. The response can be accounted for by the intrins
ic ultrasensitivity of the oocyte's MAPK cascade and a positive feedba
ck loop in which the cascade is embedded. These findings provide a bio
chemical rationale for the all-or-none character of this cell fate swi
tch.