Isoprene is a substantial contributor to the global secondary organic aerosol (SOA) burden, with implications for public health and the climate system. The mechanism by which isoprene-derived SOA is formed and the influence of environmental conditions, however, remain unclear. We present evidence from controlled smog chamber experiments and field measurements that in the presence of high levels of nitrogen oxides (NO x = NO + NO 2 ) typical of urban atmospheres, 2-methyloxirane-2-carboxylic acid (methacrylic acid epoxide, MAE) is a precursor to known isoprene-derived SOA tracers, and ultimately to SOA. We propose that MAE arises from decomposition of the OH adduct of methacryloylperoxynitrate (MPAN). This hypothesis is supported by the similarity of SOA constituents derived from MAE to those from photooxidation of isoprene, methacrolein, and MPAN under high-NO x conditions. Strong support is further derived from computational chemistry calculations and Community Multiscale Air Quality model simulations, yielding predictions consistent with field observations. Field measurements taken in Chapel Hill, North Carolina, considered along with the modeling results indicate the atmospheric significance and relevance of MAE chemistry across the United States, especially in urban areas heavily impacted by isoprene emissions. Identification of MAE implies a major role of atmospheric epoxides in forming SOA from isoprene photooxidation. Updating current atmospheric modeling frameworks with MAE chemistry could improve the way that SOA has been attributed to isoprene based on ambient tracer measurements, and lead to SOA parameterizations that better capture the dependency of yield on NO x . air pollution | anthropogenic | biogenic | particulate matter | fine aerosol

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To define prognostic impact of EpsteinBarr virus (EBV) infection in diffuse large

The epigenomic reader Brd4 is an important drug target for cancers. However, its role in cell differentiation and animal development remains largely unclear. Using two conditional knockout mouse strains and derived cells, we demonstrate that Brd4 controls cell identity gene induction and is essential for adipogenesis and myogenesis. Brd4 co-localizes with lineage-determining transcription factors (LDTFs) on active enhancers during differentiation. LDTFs coordinate with H3K4 mono-methyltransferases MLL3/MLL4 (KMT2C/KMT2D) and H3K27 acetyltransferases CBP/p300 to recruit Brd4 to enhancers activated during differentiation. Brd4 deletion prevents the enrichment of Mediator and RNA polymerase II transcription machinery, but not that of LDTFs, MLL3/MLL4-mediated H3K4me1, and CBP/p300-mediated H3K27ac, on enhancers. Consequently, Brd4 deletion prevents enhancer RNA production, cell identity gene induction and cell differentiation. Interestingly, Brd4 is dispensable for maintaining cell identity genes in differentiated cells. These findings identify Brd4 as an enhancer epigenomic reader that links active enhancers with cell identity gene induction in differentiation.