Oxygen transport in the blood is mediated by highly specialized red cells. The majority of the proteins of a red cell comprises the oxygen carrier protein hemoglobin, which is a hetero-tctrameric protein that consists of two a and two p globin chains. In humans different u- and p-like globin chains are expressed during development resulting in several different hemoglobin tetramers. The expression of different globin chains serves to facilitate the oxygen uptake by the emblYo, since embryonic hemoglobin has a higher affinity for oxygen. The genes that code for the u- and p-like globin chains reside in loci located on separate chromosomes. Many blood disorders, like u- and p-thalassemias and sickle cell anemia are the consequence of deletions or mutations of sequences in these loci and they initiated extensive research into the molecular basis of these diseases. Especially the p globin locus has served as a model system to study the regulation of multi gene loci. The five functional globin genes, 5'-s_Gy_Ay-o-P-3', are differentially expressed during development. Proper expression of these genes requires the presence of a region located 5' upstream of the s globin gene. This locus control region (LCR) contains 5 small regions that bind several trans-acting factors in an erythroid environment. The aim of this thesis is to study the role of the LCR in the regulation of the ~ globin genes ill vivo. Chapter 1 reviews the CUlTent knowledge in the regnlation of eukaryotic transcription and chromatin. Chapter 2 gives a broad introduction covering two decades of shldies conceming the regulation of the human and murine p globin genes and serves as an outline for the results that wiII be discussed in the chapters 3 to 6. Chapter 3 describes the role of EKLF in the y to p globin switching process. Chapter 4 and 5 describe experiments that show that the human and the murine LCR can only activate one globin gene at a given moment. Chapter 6 describes the characterization of intergenic transcription in the human p globin locus. Finally in chapter 7 the implications of the results presented in this thesis will be discussed.