Tautz, D., Tautz, C., Webb, D., & Dover, G. A. (1987). Evolutionary divergence
of promoters and spacers in the rDNA family of four Drosophila species. Implications for molecular coevolution in multigene
families. Journal of Molecular Biology (London), 195(3),
525-542. doi:10.1016/0022-2836(87)90181-1.