Isolated nucleic acid molecules are provided which encode Fkh.sup.sf, as well as mutant forms thereof. Also provided are expression vectors suitable for expressing such nucleic acid molecules, and host cells containing such expression vectors. Also provided are pharmaceutical compounds and methods of identifying such compounds that can modulate the immune system. In addition are provided methods for identifying proteins regulated by Scurfin (Fkh.sup.sf) and proteins that induce or inhibit Scurfin or Foxp3 expression.