Cancer has long been regarded as a genetic disease. Therefore, current theories on cancer development focus on genetic alterations affecting oncogenes or tumor suppressor genes. However, the mechanisms through which genetic alterations are induced are largely unknown. In this paper a theory will be developed which interprets cancer as a cell brain illness rather than a genetic disease. The complex comprising the centrosome, normally two centrioles and connecting filaments, was recently termed the 'cell brain', and was found to determine a cell's fate. It is through the cell brain, instead of the genes, that genetic stability and expression are maintained and regulated. Accordingly, the nucleus is regarded as a safe storage for inheriting materials (genes) that primarily act as manufacturing templates. Therefore, cancer should be regarded as a 'brain illness' of a cell, instead of a genetic disease, which is strongly supported by the latest evidence, as discussed in this paper. Such a theory serves to better clarify the confusing observations in cancer development accumulated over the last decades.