The rRNA genes present in eukaryotes are usually maintained as completely identical copies, but cases have been found where an individual organism has variation in 18S, 5.8S, or 28S rDNA sequences. I know of one case in which the question of whether or not these variant copies are transcribed was addressed: In the sturgeon (Krieger and Fuerst, J. Appl. Ichthyol. 20 (2004), 1–7). In that case, only the most common 18S rDNA variant was found to be transcribed. Does anyone know of cases in which it has been shown that the multiple variant sequences of 18S, 5.8S, or 28S rDNA are transcribed? Or if they are used in ribosomes? Thanks!