@tomrennie The genetic mutation is called DPYD. I was tested before starting Capecitabine. The test results showed I am an intermediate metabolizer, meaning that Capecitabine is to expected to be metabilized at a level between normal and poor with an increase of toxicity. My oncologist has started me on a lower dosage of Capecitabine, one pill 500 mg in the morning, and the same dosage 12 hours later. So far the only side effect I have had is some fatigue.