The fundamental mechanisms governing how cells form an identity such as becoming a muscle cell or a nerve cell are not fully understood. Multiple diseases, including cancer, have been linked to cells going down the wrong developmental path during maturation. A new study from the Perelman School of Medicine at the University of Pennsylvania suggests that the ability of a stem cell to differentiate into cardiac muscle (and by extension other cell types) depends on what portions of the genome are available for activation, which is controlled by the location of DNA in a cell’s nucleus.