  In the study of integrable non-linear $\sigma$-models which are assemblies
and/or deformations of principal chiral models and/or WZW models, a rational
function called the twist function plays a central role. For a large class of
such models, we show that they are one-loop renormalizable, and that the
renormalization group flow equations can be written directly in terms of the
twist function in a remarkably simple way. The resulting equation appears to
have a universal character when the integrable model is characterized by a
twist function.
