Schlötterer C, Hauser MT, von Haeseler A, Tautz D.

Mol Biol Evol. 1994 May;11(3):513-22. doi:10.1093/oxfordjournals.molbev.a040131

Comparative evolutionary analysis of rDNA ITS regions in Drosophila.

Average substitution rate of 1.2% per million year.