Diffuse Large B-Cell Lymphoma is a genetically heterogeneous form of cancer, with different cells in the tumour acquiring different mutations. This intra-tumour heterogeneity can have profound effects on treatment; if a clonal subpopulations is resistant to treatment, it will reform the tumour following treatment, causing the patient to relapse. We are recieving samples from several clinical trials exploring different salvage therapies for patients with relapsed DLBCL. Tumour biopsies and liquid biopsies are being collected from patients both before and after they are treated. Using these temporal sources of tumour DNA, we can identify clonal subpopulations in the tumour, and further explore how the tumour evolves and adapts in the face of a strong selective pressure such as treatment. The mutations unique to these resistant clonal subpopulations can be explored to determine which mutation(s) result in treatment resistance. We can further determine if these mutations are detectable prior to treatment, which may allow patient to be screened for resistance mutations before they are treated.