PTEN and cervical squamous intraepithelial neoplasia: While work from yeast and mammalian systems suggests that there are hundreds of genes that may cause – or protect cells from CIN, it is noteworthy that the over-expression of some classical oncogenes [e.g. KRAS; (20)] and loss of some tumor suppressors [e.g. PTEN, (21)] also leads to CIN.