RB1 and neoplasm: Specifically, animals that carry deletion in the tumor protein p53 (p53) [27,28,29,30] and/or retinoblastoma 1 (RB1) genes [27,29,30] and others that over-express the Fos Proto-Oncogene, AP-1 Transcription Factor Subunit (c-FOS) [31,32] in primitive mesenchymal cells or osteoblasts have been generated and bear different features regarding tumor latency, penetrance and the development of spontaneous metastasis (reviewed in [20]).