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The chicken egg yolk protein genes, vitellogenin (VTG) I, II, III, and the very low density apolipoprotein II (apoVLDLII), are highly expressed in hepatic parenchymal cells in an estrogen-dependent manner. Compared to the other egg yolk protein genes which respond in approximately 2-4 hours after the primary administration of estrogen, the VTGI does not respond for about 20 hours. The mechanism of activation of the egg yolk protein genes is not completely known. Of the VTG genes VTGII and III promoter regions have been isolated and characterized, whereas the promoter region of the VTGI gene has yet to be identified. Partial isolation of the VTGI gene reveals that the gene is at least 30 kb in length. The length of the other VTGs is about 20 kb. Primers specific for the clone representing the {dollar}5\\sp\\prime{dollar} most cDNA fragment corresponding to the VTGI gene shows multiple bands in a primer extension experiment. Thus the assignment of the transcriptional start site is impossible. In addition, any genomic DNA sequence isolated using this cDNA, undergoes a rearrangement event, making it impossible to identify the promoter region of the VTGI gene. The egg yolk protein genes are characterized by a lag in the appearance of mRNA by 2-4 hours for the apoVLDLII, VTGII, and VTGIII genes and about a 20 hour lag for the VTGI gene in response to the administration of estrogen. After the estrogen is depleted and the level of mRNA for the egg yolk protein cannot be detected, a secondary administration of estrogen shortens the lag period to 1-2 hours for the apoVLDLII, VTGII, VTGIII, and about 10 hours for the VTGI mRNA. This phenomenon is known as hepatic memory. Transcriptional analysis of the egg yolk protein genes by nuclear run-on assays reveals that during the both the primary and secondary responses to estrogen, the lag period is primarily due to transcriptional regulation. The mechanism(s) responsible for this primary and secondary lag still remains elusive.