The purpose of this study is to determine the proportion of patients diagnosed with Lynch syndrome in colorectal cancer patients with the loss of staining by immunohistochemistry (IHC) of any of the mismatch repair (MMR) proteins. Besides, this study aims to test the specificity and the sensitivity of detecting microsatellite instability (MSI) by next-generation sequencing, and to find out the consistency between IHC and MSI in colorectal cancer patients in China. In addition, researchers want to analyze the clinical characteristics and germline mutation of Lynch syndrome in Chinese population.

1. Detect microsatellite instability (by next-generation sequencing and PCR capillary electrophoresis) and germline mutation (by next-generation sequencing) in probands.
2. Analyze the test outcome with clinical and family information to evaluate the germline mutation status preliminarily: likely pathogenic germline mutation, variant of uncertain significance, non-pathogenic germline mutation.
3. Verify the germline mutation in blood relatives whose proband has known likely pathogenic germline mutation or variant of uncertain significance.
4. Diagnose pathogenic germline mutation and non-pathogenic germline mutation based on clinical characteristics, family information and germline mutation test outcomes (including the outcomes of probands and blood relatives). Diagnose Lynch syndrome and the pathogenic germline mutation carriers in the included population.
5. Analyze the specificity and the sensitivity of detecting microsatellite instability (MSI) by next-generation sequencing; and analyze the consistency between IHC and MSI.
6. Analyze the clinical characteristics and germline mutation of Lynch syndrome in Chinese population.

For probands, the inclusion criteria:

All of the following four points should be satisfied:

• Histological diagnosis of colorectal cancer;
• With the loss of staining by immunohistochemistry of any of the mismatch repair (MMR) proteins (MLH1, MSH2, MSH6, PMS2);
• With sufficient tumor tissue and normal tissue to test;
• Agree to provide basic information, clinical information and family history of cancer information.

For probands, the exclusion criteria:

• With at least one blood relative with known pathogenic germline mutation(s).

For blood relatives verifying germline mutation, the inclusion criteria:

All of the following three points should be satisfied:

• First- to second-degree blood relatives of probands with germline mutation(s).
• With Sufficient tumor tissue and normal tissue to test.
• Agree to provide basic information, clinical information and family history of cancer information.

For blood relatives verifying germline mutation, the exclusion criteria:

• Blood relatives who refuse to test.

The probands will be selected from colorectal cancer patients with the loss of staining by immunohistochemistry of any of the mismatch repair (MMR) proteins (MLH1, MSH2, MSH6, PMS2).

The blood relatives verifying germline mutation will be selected from whose probands have germline mutation(s).