Kappa deleting recombination excision circle (KREC) is a molecular structure that occurs as a result of the specific response of B lymphocytes to protect the immune system against a wide variety of antigens and is considered to be an indicator of early immune response, especially in immunodeficiency disease patients. KREC can be measured with molecular analysis methods to show B cell output from the bone marrow and there has been an increase in clinical studies on the subject recently. This review is focused on explaining the mechanism of KREC occurrence, the importance of KREC measurement and its role in the treatment of diseases.

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