CD3D and Allergy: Gene expression microarray analyses of these cells showed a more than a two-fold increase in expression of six of the TCR pathway genes; CD3D, CD3Z, LCK, ITK, ZAP70 and MAP4K1. Of these, ITK was selected as a putative reference gene because of its involvement in the polarization of Th2 cells and its location in a chromosomal susceptibility region for allergy, 5q31-33 [18].