Productive, persistent HPV infection requires viral entry into immature basal epithelial cells. As a result, surfaces covered with mature, intact squamous epithelium, such as the ectocervix, vagina, vulva, penis, and oropharynx, are normally resistant to HPV infection. Sites in the female genital tract that are susceptible to infection include areas of squamous epithelial trauma and repair, where the virus may access basal cells, and the immature metaplastic squamous cells that are present at the squamocolumnar junction of the cervix (see Fig. 22.12). The cervix, with its relatively large areas of immature squamous metaplastic epithelium, is particularly vulnerable to HPV infection. Other sites in the body that are vulnerable to HPV infection include the squamocolumnar junction of the anus and the squamous cells of oropharyngeal tonsillar crypts, both relatively common sites of HPV-associated cancers in individuals who practice anal or oral sex, respectively.