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 tumor mutational burden (TMB), which has shown some correlation to the efficacy of checkpoint inhibitors. Assessments of TMB are being added to many NGS assays to enable their use for patient selection in I-O clinical trials, and perhaps as companion diagnostics to these therapies. However, it is also known that TMB scores can differ significantly between assays and especially around levels that may be clinical decision points. To improve and harmonize the measurement of TMB, we have generated and characterized tumor-normal human cell lines, fragmented and sized to ctDNA fragment sizes, and determined their Blood TMB scores by a targeted NGS panel.