NF1 and papilloma: Strikingly, a large fraction of Nf1+/− mice (44%, 4 out of 9) developed papilloma, whereas no (0 out of 50) Nf1f/f (wild-type Nf1 in all tissues) or K14Cre; Nf1f/f (Nf1 knockout in keratinocytes but wild type in all other tissues) mice, here after called Nf1+/+ mice (all Nf1+/+ mice are from Fig. 1 in the previous section), showed any sign of papilloma formation at week 19 (Fig. 3a).