Mechanisms of tumorigenesis and novel cancer therapies have been investigated in cultured mutant cells, genetically engineered to develop tumors, but the behaviour of these cells in a normal tissue environment remains almost completely unknown. In September 1 Journal of Clinical Investigation, Youyan Zhang and colleagues from Indiana University School of Medicine, Indianapolis, show that normal cells can strongly modulate the growth of mutant populations in vivo and this effect should be taken into account in experiments using only a homogeneous source of tumor-prone cells.

Zhang et al. used a Nf1 mutant murine model of juvenile myelomonocytic leukaemia and performed competitive repopulation assays to quantify the proliferative advantage of hematopoietic cells that lack the NF1 tumor-suppressor gene. They found that although mutant stem cells demonstrated a growth advantage over non-mutant cells, Nf1-deficient cells did not progressively outcompete their wild-type counterparts over months of observation. The myelomonocytic leukaemia only...