“…It is not clear why SV40-transformed human epithelial cells are usually non-tumorigenic in nude mice (an exception being late-passage SV40-transformed keratinocytes; described by Brown & Gallimore, 1987), while SV40-transformed rodent epithelial cells, such as rat hepatocytes (Woodworth et al, 1988), are often strongly tumorigenic (Freedman & Shin, 1978). The fact that some SV40-transformed human epithelial cell lines, such as the 184A1 mammary line (Clark et al, 1988) and the HBL-100 mammary line (Saint-Ruf et al, 1988), can be converted to tumorigenicity by ras oncogenes (activated Ha-ras and Ki-ras respectively) strongly suggests that the parent lines are indeed incompletely transformed. Both of the thyroid cell lines (HTori-3 and HTori-5) subjected to chromosome analysis were aneuploid, which is to be expected after transformation by SV40.…”