Annexin A7 knock out mouse red blood cells show a more flat shape and have a higher osmotic resistance. The presence of annexin A7 may alter the membrane flexibility in that it supports the typical biconcave membrane shape and leads to a lower minimal osmotic resistance. Annexin A7 does not appear to have an influence on plasma membrane integrity itself as the maximal osmotic resistance value stays constant. Ca2+ and membrane skeletal proteins are known to play key roles on the red blood cell membrane shape and stability [47]. Annexins have been reported to bind directly to F-actin, we could however exclude such an activity for annexin A7. Nevertheless it might interact with other components and have a role in organizing the membrane cytoskeleton, controlling raft protein associations or influencing the ionic strength of red blood cells on its own or by interfering with other signaling pathways.