 The unpredictable nature of these events are also a main contributor to the heterogeneity observed among tumour cells.It is currently accepted that sporadic tumors (non-familial ones) are originated due to the accumulation of several genetic errors.An average cancer of the breast or colon can have about 60 to 70 protein altering mutations, of which about 3 or 4 may be "driver" mutations, and the remaining ones may be "passenger" mutations Any genetic or epigenetic lesion increasing the mutation rate will have as a consequence an increase in the acquisition of new mutations, increasing then the probability to develop a tumor.