  To explain the appearance of heterogeneities in the plasma membrane, I
propose a hypothesis which begins with the observation that fluctuations in the
membrane curvature are coupled to the difference between compositions in one
leaf and the other. Because of this coupling, the most easily excited
fluctuations can occur at non-zero wavenumbers. When the coupling is
sufficiently strong, it is well-known that it leads to microphase separation
and modulated phases. I note that when the coupling is less strong, the
tendency towards modulation remains manifest in a liquid phase that exhibits
transient structure of a characteristic size; that is, it is a microemulsion.
The characteristic size of the fluctuating domains is estimated to be on the
order of 100 nm, and experiments to verify this hypothesis are proposed.
