Scientists led by Dr. Salvador Aznar-Benitah, head of the Stem Cells and Cancer laboratory at IRB Barcelona, have described the alterations that occur during mammary gland formation when heterochromatin (the part of DNA that does not actively produce proteins) is poorly regulated. The results, which have been published in the journal Cell Stem Cell, indicate that incorrect DNA packaging makes retrotransposons (a type of transposable element originated in ancestral fragments of viruses integrated into the cell genome) more accessible.