Lymphoid cells from scid mice initiate V (D) J recombination normally but have a severely reduced ability to join coding segments. Thymocytes from scid mice contain broken DNA molecules at the TCRG locus that have coding ends, as well as molecules with signal ends, whereas in normal mice we previously detected only signal ends. Remarkably, these coding (but not signal) ends are sealed into hairpin structures. The formation of hairpins at coding ends may be a universal, early step in V (D) J recombination; this would provide a simple explanation for the origin of P nucleotides in coding joints. These findings may shed light on the mechanism of cleavage and suggest a possible role for the scid factor.