The human small nuclear RNA (snRNA)1 genes, which encode snRNAs that are involved in RNA processing reactions such as mRNA splicing, serve as prototypes for a family of genes whose promoters are characterized by the presence of a proximal sequence element (PSE) and a distal sequence element (DSE). From a transcription point of view, this family of genes is highly interesting because all of its members have very similar promoters, even though some of them are transcribed by RNA polymerase (pol) II and others by pol III. As a result, the snRNA genes have served as a model system to explore how RNA polymerase specificity is determined and, in general, to compare the pol II and III transcription machineries. This has led to the concept that the pol II and III transcription machineries use common factors, the best known of which is the TATA box binding protein (TBP). In addition, the relative simplicity of these promoters has also made them an attractive system to study how transcriptional activators perform their function.