A widely recognized difficulty of presently used methods for cDNA cloning is obtaining cDNA segments that contain the entire nucleotide sequence of the corresponding mRNA. The cloning procedure described here mitigates this shortcoming. Of the 105 plasmid-cDNA recombinants obtained per ,ug of rabbit reticulocyte mRNA, about 10%o contained a complete a-or P-globin mRNA sequence, and at least 30 to 50%o, but very likely more, contained the entire globin coding regions. We attribute the high efficiency of cloning full-or nearly fulllength cDNA to (i) the fact that the plasmid DNA vector itself serves as the primer for first-and second-strand cDNA synthesis, (ii) the lack of any nuclease treatment of the products, and (iii) the fact that one of the steps in the procedure results in preferential cloning of recombinants with full-length cDNA's over those with truncated cDNA's.

We have cloned a full-length 1.6-kilobase cDNA of a human mRNA coding for hypoxanthine phosphoribosyltransferase (HPRT; IMP:pyrophosphate phosphoribosyltransferase, EC 2.4.2.8) into a simian virus 40-based expression vector and have determined its full nucleotide sequence. The inferred amino acid sequence agrees with a partial amino acid sequence determined for authentic human HPRTprotein..Transfection of HPRTdeficient mouse IA9 cells with the purified plasmid leads to the expression of human HPRT enzyme activity in cells stably transfected and selected for enzyme activity in hypoxanthine/aminopterin/thymidine medium.Current methods of molecular biology, including the techniques of recombinant -DNA construction and cloning, rapid nucleotide sequence analysis, the design and construction of transducing vectors, and techniques oftransfection ofeukaryotic cells with foreign genes, have made it possible to clone and characterize a large number of eukaryotic genes. One gene of particular interest, not only for basic studies of eukaryotic gene regulation but also for understanding of several important human genetic diseases, is the gene encoding the enzyme hypoxanthine phosphoribosyltransferase (HPRT) (1). This enzyme catalyzes vital steps in the reutilization pathway for purine biosynthesis, and its deficiency leads to forms ofgouty arthritis and to the devastating Lesch-Nyhan disease (2,3). The HPRT locus is known to be X linked in the human and other mammalian genomes, and the availability of a cloned HPRT gene would facilitate studies ofthe organization ofa particularly interesting region of the human X chromosome and of the mechanisms of inactivation of specific and well-mapped regions of the X chromosome. Recently, we succeeded in isolating a human genomic clone containing a portion ofthe HPRT (4) by a combination of gene transfection into enzyme-deficient mouse cells and localization of human sequences by hybridization with probes of middle repetitive human (Alu) sequence, as described (4). A 15-kilobase (kb) EcoRI/BamilI fragment derived from p6B2aE2 and free ofrepetitive human sequences ,has been subcloned into the BamHI and EcoRI-sites ofpBR322 and shown to hybridize to a discrete human cytoplasmic poly(A)+RNA approximately 1.5 kb long, presumably. representing the mRNA for HPRT (4). DNA from this subcloned plasmid, called pBR1.5, was prepared by detergent lysis (5) of chloramphenicol-treated (6) transformed SF8 Escherichia coli (7) followed. by cesium chloride/ethidium bromide ultracentrifugation (8). This DNA was cleaved with EcoRI/BamrHI and the cloned insert was isolated by trough electroelution onto a dialysis membrane (9) The colonies on the replica plates were transferred to Whatman 541 paper, amplified on chloramphenicol plates overnight at 37QC by the method ofGergen et at (13), denatured and fixed to the filters, and screened for HPRT cDNA sequences by hybridization with the gel-purified insert from pBR1.5 containing the HPRT fragment free ofrepeat sequences. The filter replicas of the...

3Critical roles for estrogens in growth and development and in pathological conditions of bone, breast, and uterus are well established. Estrogens and estrogen receptor modulators bind to estrogen receptor ␣ (ER␣) and/or ER␤ to form discrete molecular complexes that exert pleiotropic tissue-specific effects by modulating the expression of target genes. Ligandbound ER functions as a key transcription factor in various molecular pathways, and modulation of ER expression levels is important in determining cellular growth potential. Recent advances have begun to illuminate the mechanisms by which cells control ER expression. Kos et al. reviewed the genomic organization of the human ER␣ gene promoter region (31). The human ER␣ gene, located on chromosome 6, was cloned in 1986 and spans approximately 300 kb; its eight coding regions are transcribed from at least seven promoters (31). Considering the complexity of the ER promoter, it is not surprising that numerous factors affect ER␣ expression. The aim of this paper is to review the molecular mechanisms by which various cellular factors modulate ER␣ expression. We have focused on highlighting the major players, including effectors of chromatin structure, hormones, and other relevant agents, and limited our discussion to findings in human systems. There are vast qualitative and quantitative data regarding whether or not, and to what extent, ER␣ is expressed in normal and neoplastic tissues; this topic is beyond the scope of this review and will not be covered. Moreover, little is currently known about the role of specific transcription factors in ER expression, and these factors will be mentioned only briefly. Henceforth, the terms ER␣ and ER will be used interchangeably; expression of ER␤ will not be discussed.

The temporal sequence of expression of human globin genes during development suggests precise regulation of these genes. Recent studies have characterized a number of DNA sequences within or flanking the human beta-globin gene which are important in its regulation and several proteins which bind to these sequences have been identified. We have found two proteins which bind 5' to the human beta-globin gene. One of these proteins, which we designate BP1, binds to two sequences, one between -550 and -527 bp relative to the cap site, the other between -302 and -294 bp. A second protein, BP2, binds to sequences between -275 and -263 bp. The binding sites for both BP1 and BP2 are in two regions which function as silencers in a transient expression assay using the human erythroleukemia cell line K562. These results and others presented here suggest that BP1 may act as a repressor protein. Negative regulation seems to be an important component of tissue and developmental specific globin gene regulation.