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DNA stains reveal that the nuclei of rod cells (top left) from a mouse’s retina have a different arrangement of DNA than ganglion cells (bottom left) or skin cells (right). Blue and red colors show where densely-packed inactive DNA called heterochromatin is located; green shows where active DNA is located. In most cells, heterochromatin is pushed to the outer edges of the nucleus. But rod cells in nocturnal mammals have heterochromatin concentrated in the center, where it acts like a lens to focus light, perhaps enhancing night vision, a new study reports. Credit: Solovei, I. et al. Cell April 17, 2009