Transcription and translation of cyclin-dependent kinase inhibitor 1 (p21<sup>Cip1</sup>) and SAM domain and HD domain-containing protein 1 (SAMHD1) were analyzed by TaqMan PCR and Western blot experiments. siRNA experiment was applied to study the role of p53 downstream gene p21<sup>Cip1</sup> in the restriction of retrovirus infection.