This valley in LUT plots in SP-D images is very consistent, presumably it is the more tightly wound “neck” region of the molecule. The CRD and neck and part of the collagen-like domain from left to right, red arrow at the low point. This means that the collagen-like domain is disproportionately elongated in terms of the whole molecule. I am aware of the numerous articles that go into great detail as to the CRD of different species and their effectiveness in innate immunity. This particular image the valley is quite pronounced. In various LUT plots, there is a valley and gradual rise in peak to the CRD. The thinness of the LUT at this point as well as the valley could indicate a tighter winding of the neck and a looser winding of the collagen-like domain, and thus help someone produce a molecular model of that region so the complete SP-D can be imaged. This image from Arroyo et al.