The single site of integration of hepatitis B virus in the human hepatocellular carcinoma cell line Hep 3B 2-1/7 was found to segregate with human chromosome 12 in somatic cell hybrids. Analysis of metaphase spreads of Hep 3B 2-1/7 following in situ hybridization with pHBV revealed integration at 12q13→q14, a location that coincides with a fragile site, fra (12q13). The possible significance of this location to the development of hepatocellular carcinomas is discusse