since pro cannot be governed it cannot have a governing category, and so is exempt from the binding theory. under this definition, the features of pro no longer conflict with the principles of binding theory. however, developments in binding theory since 1981 have presented significant challenges to the pro theorem. for example, if pro is ungoverned, then it must not be case-marked. however, in icelandic, pro appears to be case-marked, and is thus governed. more recent research attempts to characterize pro without reference to the pro theorem.