Mouse NK1⁺ T cells constitute a special subset of αβ TCR⁺ T cells that express natural killer surface receptors and are thought to play an immunoregulatory role because of their unique ability to secrete IL-4 within minutes of primary activation. The recent discovery that they recognize non-polymorphic MHC class I like ligands encoded by CD1 family genes sheds a new light on unusual aspects of their development as well as on some of the possible ways in which they might influence the regulation of the T-helper cell (types 1 and 2) classes of immune responses.