Every cell in the body has two genomes, one from the mother and one from the father. Until now, researchers have lacked the tools to examine — in a single cell –the exact readout from each genome to make RNA. Using a new technology that allows researchers to do just that, an interdisciplinary University of Pennsylvania team examined a rare disease in which these two genomes are expressed differently throughout the body, even sometimes in the same organ. They found that at the single-cell level gene expression was highly variable and quite different than expected, which is now shedding light on the molecular causes of rare diseases and perhaps the complex nature of tumors.”This is a great example of cross-school collaboration,” said co-senior author Marisa Bartolomei, PhD, a professor of Cell and Developmental Biology in the Perelman School of Medicine. Her colleagues are co-first author Jennifer M. Kalish, MD, PhD, an expert in rare growth disorders from The Children’s Hospital of Philadelphia, and co-first author Paul Ginar, an MD//PhD candidate, and co-senior author Arjun Raj, PhD, both from the School of Engineering and Applied Science at Penn, who devised the technology to image single genes in individual cells. The team published their findings in Genes & Development.