The biggest-ever map of the DNA drivers for cancer suggest a far higher number of genes may be implicated in this disease than was previously thought, a study published on Thursday says. A international consortium of scientists sequenced more than 500 genes suspected to play an important role in initiating or spreading cancer. Cancer is caused by mutations in genes. These cause abnormal cells to proliferate into tumour,although it may take years before the problem becomes apparent, by which time a tumour may number billions of cells.