title: Corrections and clarifications.
abstract: Note 21 on page 1178 of the report "Mice lacking TdT: Mature animals with an immature lymphocyte repertoire" by S. Gilfillan et al. (27 Aug., p. 1175) should have included the information that the cDNA sequence of mouse TdT was initially obtained by the authors from the European Molecular Biology Laboratory (accession number X04123), given in a 1986 paper by O. Koiwai, T. Yokota, T. Kageyama, T. Hirose, S. Yoshida, and K. Arai [Nucleic Acids Res. 14, 5777 (1986)].