@bceg1969 , I have no training in biology or medicine, so this explanation might not be perfect (or even adequate), but it's a quick answer others can clarify or correct if necessary.

You might be aware of how the DNA in your cells is a long chain of base molecules paired up in a certain order. When cells reproduce, the new cells are supposed to have the base molecules of their DNA in the same order. They usually do, but sometimes there's a biological "mistake" in the copying process that messes up the order or the way those molecules pair up.

That mistake represents a "mutation" and it will be passed on to new cells that reproduce from it, and possibly also to the offspring of the person carrying those cells. Not all mutations are harmful, but some are, and they may cause or allow the cells to grow out of control as they do in cancer.

The mutations have names based on where in the chain they occur and what life function they affect. In pancreatic cancer, the mutations you'll commonly see have names like KRAS, PALB, BRCA1, BRCA2, ATM, and others. The ATM name comes from a condition called Ataxia-Telangiectasia that may develop in a child if both parents have an ATM mutation that is passed on to the child.

Different tests can identify whether you have these changes in your blood and/or in your tumor, and can often identify whether they were inherited from one of your ancestors or caused by something in your environment (like exposure to radiation, asbestos, chemicals, etc).

Some of the systemic cancer treatments (chemotherapy, immunotherapy, targeted therapy, as pills or IV infusions) are believed to be more effective in patients with certain mutations, and systemic treatments are specifically designed to target cells with those mutations. The ideal situation is to have a drug which attacks and kills only cancer cells while leaving normal, healthy cells alone. Researchers are making good progress on this, but are still far from the holy grail of an answer.

Cancer cells try really hard to stay alive, and may adapt to their harsh new environment by developing new mutations that provide resistance to the drugs that are attacking them. Drug resistance typically requires switching to a different treatment.