Human hereditary tumors in vivo are known to show tissue specificity. Increased endoreduplication with tetraploidy has been shown to occur in cultures established only from tissue containing epithelium (skin and colonic polyps) from patients with the Gardner syndrome. Cultures established from blood (short-term lymphocyte and lymphoid suspension) and connective tissue (subcutaneous tissue, lipoma, mesentery, and sebaceous cyst) from the same patients did not show increased tetraploidy. These observations demonstrated that tetraploidy as an expression of the gene for the Gardner syndrome was cell-specific in vitro and added further evidence that increased tetraploidy could be used for detection of the Gardner gene in families at risk.