The understanding of the phenotypic variability of genetically homogeneous disorders represents a major challenge. In beta thalassemia, the beta globin gene is affected by a variety of mutations. The group of patients to be analyzed here is homozygous for a splice site mutation that is common in the Middle East. In contrast to this genetic homogeneity, the spectrum of the clinical phenotype ranges from mild anemia to most severe, transfusion dependent anemia. We will use a genetic linkage approach to identify modifying factors and by analyzing the efficiency of an mRNA surveillance mechanism that is referred to as nonsense-mediated decay and represents a candidate genetic modifier of beta thalassemia and other genetic disorders.