Isolated nucleic acid molecules encoding T-bet, and isolated T-bet proteins, are provided. The invention further provides antisense nucleic acid molecules, recombinant expression vectors containing a nucleic acid molecule of the invention, host cells into which the expression vectors have been introduced and non-human transgenic animals carrying a T-bet transgene. The invention further provides T-bet fusion proteins and anti-T-bet antibodies. Methods of using the T-bet compositions of the invention are also disclosed, including methods for detecting T-bet activity in a biological sample, methods of modulating T-bet activity in a cell, and methods for identifying agents that modulate the activity of T-bet.