Practice question

Recombination Activating Genes RAG1 and RAG2 mediate VDJ recombination of immunoglobulin and T cell receptor loci, essential for lymphocyte development. In thymus, double negative thymocytes rearrange TCR beta, then become CD4 CD8 double positive at cortex where both co receptors are coexpressed before positive selection. Without RAG1, TCR cannot form, thymocyte development arrests at double negative to double positive transition, eliminating double positive population. Single positive CD4 or CD8 also fail to develop, leading to severe combined immunodeficiency lacking mature T cells.