Influenced by environment and genetics, the mechanism of T cell receptor (TCR) repertoire generation and the role of parameters influencing the process are poorly understood. In a recent study published in PNAS, researchers specifically study how genetic factors impact TCR diversity by analyzing the TCR repertoires of 3 pairs of identical twins. Although the researchers found that the overlap between the TCR repertoires of twins was similar to the overlap between non-related individual TCR repertoires, the researchers identified key processes in TCR repertoire generation for which genetics played key roles. What elements of final TCR repertoires are exclusively determined by genetic factors?