RB1 and papilloma: There is precedence for these findings, as studies that were done in cottontail rabbit PV (SfPV1, also called CRPV1) found E7 to be essential for promoting disease (44, 45), but that E7’s ability to interact with RB1, albeit through an LXCXE sequence (46), is not essential for inducing papillomas in experimentally infected rabbits (47).