- Interleukin 12 signaling in T helper type 1 (Th1) cells involves tyrosine phosphorylation of signal transducer and activator of transcription (Stat)3 and Stat4. Jacobson, N.G., Szabo, S.J., Weber-Nordt, R.M., Zhong, Z., Schreiber, R.D., Darnell, J.E., Murphy, K.M. J. Exp. Med. (1995)