Analysis of the transcriptomes of different organisms has demonstrated that a single gene can have multiple transcripts. The sources of transcriptional vari-ability are the alternative promoters, polyadenylation sites, splicing, and RNA editing. A comparison of the organisms of different taxa has demonstrated that the complexity of organization during evolution arises not due to an increase in the number of protein-coding genes. The greatest variability of transcripts is specific to the nervous and germinal systems. A variety of mechanisms provid-ing for the complexity of the transcriptome ensures a precise and coordinated regulation of organ-specific functions through a combination of cis-acting el-ements and trans-acting factors. The D. melanogaster sbr (Dm nxf1) gene has proven to be an excellent model for investigating mechanisms potentially lead-ing to the emergence of multiple products with various functions.