CDKN2B and melanoma: During frog husbandry, spontaneous nevi and melanoma was observed in various genotypes of Xenopus tropicalis at 18 months of age, including cdkn2b-/-/tp53-/-, cdkn2b+/-/tp53-/-, cdkn2b+/+/tp53-/-, cdkn2b-/-/tp53+/-, cdkn2b+/-/tp53+/-, cdkn2b+/+/tp53+/-, and cdkn2b-/-/tp53+/+ individuals.