# Why are the OV and VO blocks of the CIS 1RDM zero?

I know the reduced density matrix has the form: $$\gamma(\alpha,\beta) = \langle\Psi_N|c^{\dagger}_{\alpha} c_{\beta}|\Psi_n \rangle$$ but when $$\Psi_N$$ is the configuration interaction singles (CIS) wavefunction, why are the occupied-virtual (OV) and virtual-occupied (VO) subblocks zero? I see multiple references state that it is due to Brillouins Theorem, but I don't entirely see that connection.