Cajal Bodies are one of many specialised organelles contained in the eukaryotic cell nucleus, and are involved in a number of functions, including regulation of replication-dependent histone gene transcription. In normal diploid cells their number varies between 0 and 4 depending on the cell cycle phase, although in cancer cell lines their number is extremely variable and it has been suggested that it correlates with cell ploidy. Here we show that in mammalian cells, as in Drosophila, two distinct though functionally related bodies exist: a histone gene locus body and a Cajal Body. The first one can be detected using FLASH or NPAT as markers while the second is labelled using antibodies against Coilin. Only the number of FLASH/NPAT histone gene locus bodies correlates with ploidy and only these organelles appear to be regulated during the cell cycle. Finally, we show that the two organelles completely co-localize during the S phase of the cell cycle.