RAG1 is an abbreviation for recombination activating gene-1 which is the cornerstone in the process of V(D)J recombination. The process enables our immune system to produce an unlimited number of antibodies and T-cell receptors to fight almost any infection. RAG1 mutations are greatly involved in the development of primary immunodeficiencies (PID) including the early-onset severe combined immunodeficiency and Omenn Syndrome. Additionally, RAG mutations are found in delayed-onset cases of combined immunodeficiency with autoimmunity and/or granulomas. The early-onset category occurs due to severe immunodeficiency and the delayed-onset one occurs due to immune dysregulation. Furthermore, RAG1 abnormalities are found in several hematological neoplasms such as acute lymphoblastic leukemia (ALL), chronic myeloid leukemia (CML), myelodysplastic syndrome (MDS) and lymphoma. The mechanism by which RAG1 contributes to different types of cancer is not fully understood. In this review, we display the role of RAG1 in the different diseases digging to understand how it works and whether it can be a promising biomarker for the progression of some of these conditions.