In Immuno-Oncology, the goal is to enable a patient’s immune system to locate and eliminate cancerous cells. The immune system may normally protect against cancer – except, when cancers manage to evade it – since immunocompromised individuals have a higher likelihood of developing cancer. Checkpoint inhibitors, which have recently improved the prognosis for a significant number of cancer patients, work by blocking inhibitory molecules such as PD-L1 and CTLA-4 that may be allowing cancers to evade the adaptive immune system. However, by taking the brakes off the immune system, the use of checkpoint inhibitors is associated with a significant risk of developing autoimmune disease. Additionally, frequently checkpoint inhibitors are not effective. In order to maximize the safety and efficacy of checkpoint inhibitors, it is beneficial to identify patients who are likely to respond to their use. Since the adaptive immune system can detect changes to proteins, it is thought that the number of somatic mutations that lead to such changes may be correlated with efficacy. The metric of non-silent somatic mutations per megabase of coding DNA has been termed Tumour Mutational Burden (TMB), which has shown some correlation to the efficacy of checkpoint inhibitors. Assessments of TMB are being added to many NGS assays order to enable their use for patient selection for clinical trials and perhaps later as companion diagnostics. However, it is also known that TMB scores can differ significantly between assays and especially around levels that may be clinical decision points.