Here's the deal - we know it's 'chance' (not really luck) which part of your DNA is damaged by any given environmental effect. However, some DNA is more important than others. In general, a little damage is not a problem and is easily 'taken care of' by the cell which caries that DNA - either by literally repairing the DNA, silencing that DNA's function, killing itself, or asking other cells to help kill it.<p>We know quite clearly that cancer is (generally) caused by a set of mutations - not necessarily in an order, but some orders are not successful. There are four or five genes which keep social order amongst the other genes. If you silence all of these, you get cancer. Chance has it's role in the roll of dice for which DNA gets damaged, but all the other parameters can be changed too - how many sided the dice are, how often the dice get rolled, and whether all the cells in the same tissue have correlated dice-rolls.<p>Cells that deviate from what they're supposed to do are either (in order), repaired, silenced, voluntarily commit suicide, or are killed. There are proteins (genes) that are the final judges for each of these processes - and have 'go, no-go' power. Only if all of these judges are killed do you get a cell that can do anything it wants - like replicate uncontrollably to the detriment of the host ('cancer'). Thus the statistics of getting cancer roughly follow the idea that you have to get random DNA modifications of those exact 5 genes, in a single cell. Lots of things can increase your random modification rate (UV, smoke, radiation, etc). Some of these things correlate though - and again, what hurts one cell, might hurt its neighbor just as bad. They're not entirely independent events. For example, losing your DNA repair machinery (this is what HPV does - it silences your DNA repair machinery) amps up the baseline mutation rate and makes further mutations more likely (dependent correlations then arise).<p>The Brca gene that has caused so much controversy in patent law (whether a test for its existence could be patented) and indicates whether a person might or might be susceptible to breast cancer, is the master repair technician of the cell. In people who have this gene in working order, the Brca gene signs off on whether the cell is in need of repair. But if the Brca is not it working order, cells that are in need of repair might not get it, and instead are allowed to more freely operate under non-optimal internal conditions. If you are missing or have a mutated version of Brca, you are missing one of the checkpoint processes.<p>So again, we quite clearly know of a handful of genes which do most of the master regulation of a cell's job - and if these jobs go unfulfilled - by having their blueprints be damaged by the environment - you have fewer and fewer mechanisms to prevent that single cell from runaway growth.