At D < D eq , the repulsive force increases with decreasing membrane separation (Figures 2 and 3) , and the slope reflects the rigidity of the proteinÀligand complexes. The slopes of the curves obtained with the 6-repeat and 7-repeat forms, as well as with DC-SIGN, 18 are indicative of relatively rigid protein layers and hence relatively rigid necks ( Figure 3A,B) . By contrast, the forceÀdistance curve measured with the 5-repeat form at D < D eq is much shallower and does not increase steeply until D < 4À5 nm ( Figure 3C ), which is the approximate diameter of a folded CRD. 29 The forceÀdistance profile at D > 4 nm is similar to flexible polymers 30 and suggests that the neck region is unstructured in a significant population of the immobilized 5-repeat protein. The difference between the 5-repeat and other forms of DC-SIGNR and DC-SIGN is attributed to the loss of rigidity conferred by the tetramer. Tetramer dissociation into dimers could also lead to greater protein inactivation under applied force. This exception, relative to the behavior of the stable 6-and 7-repeat tetramers, provides evidence that the rigid neck region and relatively steep force profiles at D < D eq ( Figure 3A,B) requires stable tetramers.