However, the possibility that a relapse after the discontinuation of TKI administration may be due to the persistence of leukemic stem cells (LSCs) that are insensitive to TKIs renders even the most sensitive PCR-based assay for monitoring BCR::ABL1 transcript levels unable to monitor the residual quiescent CML LSCs. The gene discussed is BCR; the disease is chronic myelogenous leukemia, BCR-ABL1 positive.