Jonathan Widom, 55, died July 18 of an apparent heart attack. He was a professor of Molecular Biosciences in the Weinberg College of Arts and Sciences at Northwestern University. Widom focused on how DNA is packaged into chromosomes — and the location of nucleosomes specifically. Colleagues said the work has had profound implications for how genes are able to be read in the cell and how mutations outside of the regions that encode proteins can lead to errors and disease.