A tumour suppressor gene is a gene that reduces the probability that a cell in a multicellular organism will turn into a tumor cell. A mutation or deletion of such a gene will increase the probability of the formation of a tumor. Unlike oncogenes, tumor suppressor genes generally follow the 'two-hit hypothesis', which implies that both alleles that code for a particular gene must be affected before an effect is manifested. This is due to the fact that if only one allele for the gene is damaged, the second can still produce the correct protein. However, there are cases where mutations in only one allele will cause an effect. A notable example is the gene that codes for p53. Tumor suppressor genes, or more precisely, the proteins for which they code, either have a dampening or repressive effect on the regulation of the cell cycle or promote apoptosis, and sometimes do both. This book presents new and important research from throughout the world.