CTSK and chondrogenic neoplasm: To further understand why Lkb1-deficient mice developed osteogenic tumor, but not cartilage tumor, we examined expression of pErk in Lkb1-deficient Ctsk+ cells by immunostaining and Western blot and found that Lkb1 deletion had no obvious effect on inhibition of Erk signaling (Supplemental Figure 7, A, B, and C; see complete unedited blots in the supplemental material) and production of Ihh and Pthrp (Supplemental Figure 7D).