S4U (or 4-thiouracil in cells that express UPRT) is rapidly uptaken by cells, and s4U-RNA can be enriched with activated disulfide chemistry (e.g. HPDP-biotin [Cleary 2005], MTS-biotin [Duffy 2015], or MTS-resin [Duffy 2018]) or identified by enrichment-free conversion chemistry (Herzog 2017, Schofield 2018). Advantages: activated disulfide chemistry is both efficient and reversible, and enrichment-free chemistry offers single-nucleotide resolution.
	BrUTP is added to isolated nuclei for rapid uptake and short RNA labeling (Core 2008), while BrU is uptaken by cells at a slower rate and is applicable for pulse-chase experiments (Tani 2012). BrU-RNA is enriched using anti-BrdU antibodies. Advantages: BrU metabolic labeling methods are widely used and well validated. BrU is also compatible with immunostaining (Wansink 1993).
	EU is rapidly uptaken into cells, and EU-RNA can be enriched with click chemistry (azide-conjugated biotin) or used for live-cell nascent RNA labeling (Jao 2008). Advantages: 5-EU enrichment chemistry is covalent (although irreversible) and compatible with live-cell imaging.
	Biotin-NTPs are added in four separate reactions to isolated nuclei for rapid uptake and short RNA labeling, although all four biotin-NTPs can be added to the same reaction (Kwak 2013). Biotinylated RNA is enriched with streptavidin-coated magnetic beads. Advantages: Single base-pair resolution is achieved for the precise localization of RNA polymerase.