[…] in the pro-drop languages the subject position may be ungoverned, thus accepting PRO, while in the non-pro-drop languages, this position is invariably governed.
- Chomsky (1993), a pag.256

Assume "R" to be, in effect, a rule of Affix-movement. If "R" applies in the PF-component, then AGR governs the subject position at S-structure and at LF . If “R” applies in the syntax, then the resulting S-structure is (11): (11) NP [VP V-INFL…]. In (11), AGR (in INFL) does not govern the subject position at S-structure or LF. Therefore PRO may appear in this position. Suppose that we now take the pro-drop parameter to be (12): (12) "R" may apply in the syntax. The pro-drop languages accept this option [...] The non-pro-drop languages reject the option (12).
- Chomsky (1993), a pag.256-257