Thus, this is consistent with the availability of trans-acting factors in placental tumor (but not nonplacental MCF-7) cells that can access the hCS 3′-Enh region, which presumably has a relatively open (chromatin) configuration in the context of transfected plasmid DNA, but cannot access the equivalent sequences in the endogenous hCS gene loci. A difference in the chromatin organization of the hCS/GH-V genes that affects accessibility of regulatory regions, including 3′-Enh sequences, could also contribute to the lower levels of expression detected in human choriocarcinoma cells versus HTP. The gene discussed is GH2; the disease is placenta neoplasm.