Cajal bodies are small nuclear organelles first described nearly 100 years ago 
by Ramón y Cajal in vertebrate neural tissues. They have since been found in a 
variety of animal and plant nuclei, suggesting that they are involved in basic 
cellular processes. Cajal bodies contain a marker protein of unknown function, 
p80-coilin, and many components involved in transcription and processing of 
nuclear RNAs. Among these are the three eukaryotic RNA polymerases and factors 
required for transcribing and processing their respective nuclear transcripts: 
mRNA, rRNA, and pol III transcripts. A model is discussed in which Cajal bodies 
are the sites for preassembly of transcriptosomes, unitary particles involved in 
transcription and processing of RNA. A parallel is drawn to the nucleolus and 
the preassembly of ribosomes, which are unitary particles involved in 
translation of proteins.