One focus of study in generative syntactic theory concerns the constraints on the distribution of 'empty categories,' syntactically empty positions, which are either derived through movement, as in (1), or are base-generated, as in (2).
(1) Who do you think [e] left? (2) [e] salio. 'He/she left.'
One of the goals of government-binding theory is to explain the distribution of both the empty non-pronominal, or 'trace,' in (1) and the empty pronominal, or pro, in (2) through general and universal principles. One such principle is the Empty Category Principle, or ECP, in (3).
(3) The Empty Category Principle [e] must be properly governed