ABSTRACT: To test gene expression changes of human cancer cells and mouse surrounding tissue cells during tumor progression, 4 different types of cancer cells (MDA-MB231Br3, PC14Br4, KM12M, A375SM) were injected into mouse brain, skin and orthotopic sites. RNAs containing human cancer cells and mouse surrounding tissue cells were extracted and hybridized into human and mouse arrays at the same times and it revealed the brain microenvironment induced complete reprogramming of metasized cancer cells, resulting in a gain of neuronal cell characteristic , mimicking neurogenesis during development. Human in vivo: 4 KB, 4 KS, 3 KC, 5 MB, 5 MF, 4 MS, 3 PB, 3 PL, 4 PS, 5 AB, 5 AS , 1 BR_N_PC Mouse in vivo: 4 KB, 4 KS, 3 KC, 5 MB, 5 MF, 4 MS, 3 PB, 3 PL, 4 PS, 5 AB, 5 AS , 3 BR, 3 PC_BRM, 1 BR_PCM