The thymocyte subsets of a Tcf1−/− control mouse without a tumor show severely reduced Wnt-activity in ISP and DN3 thymocyte subsets compared to the Tcf1+/− control mouse (Mean Fluorescence Intensity [MFI] of 385 and 104 compared to 874 and 635 in control ISP and DN3, respectively), indicating a strongly diminished nuclear response to Wnt signals due to the Tcf1 deficiency. The gene discussed is TCF7; the disease is neoplasm.