Neoplastic and nonneoplastic B cells within tumor specimens engraft with differing relative efficiencies. The fraction of B cells that are clonal is estimated from the fraction of IGHV sequencing reads that match the specimens’ previously determined sequence. To assess the relative engraftment of clonal and nonclonal B cells, the entire omentum (with fragments of associated organs) was harvested from mice implanted with tumors M, W, J, and L. 3 mice with each xenograft were harvested at 2 weeks. DNA was prepared from these fresh specimens. The fraction of IGHV sequence reads that were aligned to the established clonal IGHV sequence for that tumor was determined (sequencing studies were performed in duplicate for all specimens; each symbol represents the results for individual assays performed on the omentum and all associated organs from an individual mouse).