The present invention provides a method of reducing levels of at least one target protein in a cell. The cell is contacted with a first agent and a second agent. The first agent reduces synthesis of the target protein, e.g., by reducing levels of the mRNA of the target protein or inhibits translation of the mRNA. The second agent accelerates degradation of the target protein. The first agent may contact the cell before, after or simultaneously with the second agent. The first agent and the second agent may be in separate delivery vehicles, or in a single delivery vehicle. The first agent may be an RNAi (RNA interference) molecule, such as a small interfering RNA (siRNA), a small hairpin RNA (shRNA) or a microRNA (miRNA). The second agent may be a chimeric polypeptide containing a ubiquitin ligase polypeptide and a target protein interacting domain. The ubiquitin ligase polypeptide can be an E3 ubiquitin ligase, including, but not limited to, an SCF polypeptide, a HECT polypeptide and a UBR1 polypeptide. In one embodiment, the SCF polypeptide is an F-box polypeptide.