[…] the pronominal element P has no properties beyond those of a minimal pronominal element: namely, the features person, number and gender, which must of course match those of the coindexed antecedent in a well-formed LF-representation. We will use the notation "PRO" for the collection of these features, with some choice of values.
- Chomsky (1993), a pag.20

PRO […] is ungoverned; its antecedent (if there is one) has an independent θ-role, as does PRO […].
- Chomsky (1993), a pag.56

If α is an empty category, then α is PRO if and only if α is ungoverned […].
- Chomsky (1993), a pag.60