Biological systems can behave as siblings in several ways, including by borrowing something and never giving it back. That appears to be what the human immune system did with a protein that now helps bind and regulate the subunits that make up antibodies, according to a multi-institute research collaboration. They found that, before the immune system evolutionarily co-opted it, the protein originally belonged to gene family responsible for directing cells to move to the right location at the right time to address specific functional needs.