NBT-dye reduction of PBHP myeloid colonies in agarose culture. These are photomicrographs at low magnification (2× final magnification) where each dark spot represents a colony, containing strongly oxidase positive cells that have reduced NBT to formazan. (A) Colonies derived from normal CD34⁺ progenitor cells showing positive NBT-dye reduction to blue-black formazan. (B) Colonies derived from p67^phox deficient CGD patient CD34⁺ cells transduced with MFGS-p67^phox clone 20 exhibiting strong NBT-dye reduction of many colonies. (C) Colonies derived from sham transduced patient CD34⁺ cells, which exhibit only weak background NBT-dye reduction.