FBXW7 and neoplasm: Next, we used digital droplet PCR (ddPCR) to test for loss of the second copy of Nf1, Fbxw7 and/or Trps1 in tumors from the above-described cohorts and in tumorsphere cultures (with less stromal contamination), which would be expected if any of these genes were functioning as a recessive tumor suppressor in this context.