There are oncogenes, genes sequences known to cause cancer, and there is also epigenetics. Epigenetics says that your environment and even thoughts regulate gene expression. Just because you have it doesnt mean it will ever express itself. Genetics is a very complicated system of 3 billion base pairs constantly being edited by proteins. Some sequences upregulate, some downregulate. UV rays from the sun, for example, create Thymine dimers (separates the DNA and damages it with long enough exposure so that when there are 2 T's in sequence, it separates the pair of T's from the rest of the DNA. This damage is usually fixed by proteins that relentlessly edit and repair, so there are tons of things that cause damage, and often times it has to do with age, and how well the body repairs damaged sequences.