The TK transgene is expressed only in the spleen and thymus.

Western blot analysis of protein lysates derived from specified TK⁺ founder line tissues were probed with a rabbit polyclonal anti-TK antibody. N signifies the negative control (HU-143B human fibroblast cell line that is TK⁻). P denotes the positive control (AKR murine leukemia cell line transfected with the TK cDNA).