Quantitative Analysis of histone exchange. S. cerevisiae H2B-TAP cells were grown in isotopically light media (12C6-Arg) while an arginine auxotrophic strain was grown in isotopically heavy (13C6-Arg) media. Cultures were chemically cross-linked with formaldehyde, harvested independently, mixed 1:1 by cell weight, and cryogenically co-lysed. Chromatin was sheared to ~1 kb and affinity purified on IgG coated Dynabeads. Histones were resolved by SDS-PAGE and the percent light peptides were measured by mass spectrometry. Depending on the level of in vivo cross-linking, histones will dissociate and re-associate with the purified chromatin. This exchange can be monitored by measuring the incorporation of isotopically heavy histones (red circles). Actively transcribing chromatin is more loosely packaged and will undergo histone exchange more readily. Silent chromatin is more densely packaged and is less likely to undergo histone exchange.