Demonstration of established bone marrow and liver metastasis after intravenous injection of 5 × 10⁴ NXS2 cells by histology and tyrosine hydroxylase RT-PCR. Mice (n = 8) were killed 5 days after intravenous injection with 5 × 10⁴NXS2 cells. Bone marrow (A and B) and liver (C and D) specimens were analyzed by tyrosine hydroxylase– nested RT-PCR (A and C, top) and histology (B and D). The presence of a TH2 signal indicates NXS2 infiltration into bone marrow (A) or liver (C). GAPDH was amplified with probes lacking a TH2 signal to prove cDNA integrity (A, bottom). A sensitivity of one tumor cell in 10⁶hepatocytes was established with tyrosine hydroxylase–nested RT-PCR of NXS2 cells in liver tissue was established with reciprocal tumor to liver cell ratios of 1:10⁴ to 1:10⁷ (C, lanes 9 to 12). Paraffin-embedded sections of bone marrow and liver specimen were stained with hematoxylin/eosin. Arrows indicate focal tumor cell infiltrates photographed at 1,000 × magnification (oil immersion).