 These distinct receptors are formed by process of V(D)J recombination gene rearrangement stimulated by RAG1 and RAG2 genes.This process is error-prone, and some thymocytes fail to make functional T-cell receptors, whereas other thymocytes make T-cell receptors that are autoreactive.If a functional T cell receptor is formed, the thymocyte will begin to express simultaneously the cell surface proteins CD4 and CD8.