Effect of Genetic Polymorphism on Capecitabine-Induced Toxicity in Egyptian Cancer Patients

This study is looking at how genetic differences between patients affect their risk of side effects from capecitabine, a common oral chemotherapy drug, in Egyptian cancer patients. The goal is to help doctors personalize chemotherapy dosing in the future. **You may be eligible if...** - You have been diagnosed with breast cancer or colorectal cancer - You are currently receiving capecitabine-based chemotherapy - You are over 18 years of age - Your general health level allows participation (ECOG score 0-2) **You may NOT be eligible if...** - You are also being treated with irinotecan (another chemotherapy) - You have problems with your upper digestive system that affect how you absorb pills - You have a condition causing chronic diarrhea, such as ulcerative colitis or Crohn's disease - You have malabsorption syndrome or cannot take oral medication Talk to your doctor to see if this trial is right for you.