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Due largely to the inability to accurately quantify and characterize de novo deletion events, the mechanisms underpinning the pathogenic expansion of mt DNA deletions in aging and neuromuscular disorders remain poorly understood. Here, we outline and validate a new tool termed ' Digital Deletion Detection' (3 D) that allows for high-resolution analysis of rare deletions occurring at frequencies as low as 1 × 10−8. 3 D is a three-step process that includes targeted enrichment for deletion-bearing molecules, single-molecule partitioning of genomes into thousands of droplets for direct quantification via droplet digital PCR, and breakpoint characterization using massively parallel sequencing. Using 3 D, we interrogated over 8 billion mitochondrial genomes to analyze the age-related dynamics of mt DNA deletions in human brain tissue. We demonstrate that the total deletion load increases with age, while the total number and diversity of unique deletions remain constant. Our data provide support for the hypothesis that expansion of pre-existing mutations is the primary factor contributing to age-related accumulation of mt DNA deletions. [ABSTRACT FROM AUTHOR]