Fzd6, but not Fzd9, gene ablation delays Eμ-TCL1–induced tumor growth. Eμ-TCL1 mice were crossed with Fzd9^−/− (A, n = 40) and Fzd6^−/− mice (B, n = 18) to obtain Eμ-TCL1^+/− animals either heterozygous for Fzd6^+/− or homozygous Fzd6^−/−. The fraction of CD5⁺CD19⁺ cells in peripheral blood was determined by flow cytometry at the indicated times. Disease-free survival was defined as having less than 4% peripheral blood CD5⁺CD19⁺ cells. Disease-free survival was significantly longer in mice lacking Fzd6 as determined by log-rank test (P < .001).