Unique molecular identifiers (UMIs) have become a vital aspect of next-generation sequencing and are used for preserving quantitative information by removing PCR/sequencing errors and duplicates. Here, we integrated UMIs with our SMART® technology to detect genuine low-frequency events in full-length variable regions of BCR and TCR genes. The results showed detection sensitivity of rare clones at 0.001% and reproducibility for low amounts of total RNA (10 ng). Our technology can be used to observe clonal selection and hypermutation events in rare clonotypes found in blood and tumor tissues.