This observational study aims to investigate the role of histidine and its transporter SLC15A3 in modulating the sensitivity of colorectal cancer to immunotherapy. By analyzing the expression of SLC15A3 in tumor/normal colonic tissues from patients with colorectal cancer and assessing serum histidine metabolic levels, the study seeks to identify potential targets associated with therapeutic resistance and explore possible intervention strategies to improve immune checkpoint blockade treatment efficacy.

Inclusion Criteria:

1. Participants aged ≥18 years and ≤100 years.
2. Patients pathologically diagnosed with colorectal cancer based on colonoscopy or surgical specimens and biopsy examination; or patients who previously received PD-1 monoclonal antibody immunotherapy for colorectal cancer.

Exclusion Criteria:

1. Age <18 years.
2. Presence of poorly controlled metabolic diseases, including hypertension, diabetes mellitus, hyperlipidemia, hyperuricemia, or hyperthyroidism.
3. Presence of other severe gastrointestinal diseases, including inflammatory bowel disease, ischemic bowel disease, familial adenomatous polyposis, liver cirrhosis, MUTYH-associated polyposis (MAP), Lynch syndrome (LS), or Peutz-Jeghers syndrome (PJS).
4. History of other malignant tumors, or pathological diagnosis of colorectal inflammatory polyps, hyperplastic polyps, neuroendocrine tumors, neuroendocrine carcinoma, or mixed neuroendocrine-non-neuroendocrine neoplasms.
5. History of neurological or psychiatric disorders, such as epilepsy or depression.
6. Unqualified specimens, including hemolyzed serum samples or tissue samples that were not properly preserved in time.

Patients with pathologically confirmed colorectal cancer who received care at the Department of Gastroenterology, Department of Oncology, or Department of General Surgery at Renji Hospital, Shanghai Jiao Tong University School of Medicine.