The molecular pathology that results in hemophilia B has been determined by many studies to be highly variable. Several point mutations have been identified in a 40-bp region within the Factor IX promoter. These include mutations at nucleotides -20, -6, +8, and +13. Here we report an A to T transversion at position -5 of the Factor IX gene. There is strong circumstantial evidence to support an important role for this region of the Factor IX promoter in the developmental regulation of the gene through the binding of specific transcription factors.