rRNA genes in the human genome

Larry Moran has an excellent discussion of the organization of rRNA genes in the human genome.  It's an old post, but a good one.

If you're involved in RNA-Seq, you should read it.  Dr. Moran's discussion makes it clear why it is necessary to map reads to a synthetic rRNA-ome before doing full-genome alignment -- if you want to accurately count, or filter out, rRNA reads (which can predominate in RNA-seq datasets).