The pre-T cell receptor (pre-TCR), formed by a TCRβ chain (rearranged TCRβ gene) and by the disulfide-linked invariant pre-TCRα chain (pTα chain; codified by the PTCRA gene, localized at 6p21.2, and really a very suspect candidate gene for SCID) has an essential role during T lymphopoiesis in thymic microambient at the stage of large pre-T cell [57]. Here, PTCRA is linked to severe combined immunodeficiency.