The portion of the cervix located at the top of the vagina is covered by squamous epithelial cells; the cervical canal is lined with columnar cells. The border between these two types of cells is called the transformation zone and is located around the opening of the cervix. Cervical dysplasia or cancer typically begins in the transformation zone. Cells in this zone are very sensitive and may respond to changes in pH, toxins produced by the vagina’s microflora or circulating in the bloodstream, substances in a sex partner’s semen, douches, contraceptive foams, infections such as genital herpes or warts, or an IUD. The transformation zone constantly shifts its position as women age and may even move inside the cervical canal.