A class of nuclear body, first seen after silver staining by Ramon y Cajal in 1903, enriched in small nuclear ribonucleoproteins, and certain general RNA polymerase II transcription factors; ultrastructurally, they appear as a tangle of coiled, electron-dense threads roughly 0.5 micrometers in diameter; involved in aspects of snRNP biogenesis; the protein coilin serves as a marker for Cajal bodies. Some argue that Cajal bodies are the sites for preassembly of transcriptosomes, unitary particles involved in transcription and processing of RNA. [ http://www.ncbi.nlm.nih.gov/pubmed/10944589 http://www.ncbi.nlm.nih.gov/pubmed/7559785 http://www.ncbi.nlm.nih.gov/pubmed/11031238 http://www.neurolex.org/wiki/nlx_subcell_090901 ]