This study looks at how having low muscle mass, called myopenia, affects the side effects of cancer medicine in older people with advanced colorectal cancer. Colorectal cancer is a type of cancer that starts in the colon or rectum. The study focuses on people who are getting a treatment called 5-Fluorouracil (5-FU), which is a type of chemotherapy. Researchers want to understand if a person's genes might change how myopenia affects the side effects from chemotherapy.

• The study is for older adults with newly diagnosed advanced colorectal cancer.
• Participants must be able to speak English or Spanish for completing questionnaires.
• People with significant weight changes or treated brain metastases can join.

Participants should not be in hospice care or receiving other experimental drugs. The study does not allow people who have had chemotherapy for advanced cancer before. Those interested need to be willing to sign a consent form and understand what the study involves. The study aims to help doctors understand treatment effects better and improve care for patients with colorectal cancer.