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Notes: The particulate material of aquaeous homogenate of forebrain was separated by zonal centrifugation in a continuous 0.4-1.2 m-sucrose gradient after sedimentation at 100,000 g to eliminate the soluble material. Based on the absorbance at 280 nm, four major peaks were obtained from adult normal mice corresponding to 1.1 m (A), 0.68 m (B), 0.35 m (C), and 0.12 m (D) sucrose. The two smaller and lighter peaks, C and D, were not present when purified myelin was separated by the same procedure. B consisted of pure compact myelin; A was made of vesicles, sometimes with a double membrane. Throughout development, the myelin peak shifted from 0.58 m in young animals to 0.70 m in very old ones. Moreover, the myelin peak B drastically increased during development, as compared with peak A.-In the Trembler, the profile was close to normal, with a slightly higher yield of myelin which also peaked at a higher density. In the quaking, there were only two shoulders in the myelin density range at 0.68 m and 0.75 m; in the Jimpy only a faint shoulder was seen, at approximatively 0.67 m. In the shiverer, B was absent and only an A peak was present, at approximatively 0.85-0.90 m, which contained non-compact lamellar membranes and myelin figures with an abnormal major dense line. In the mid (an allele of shiverer) the density profile resembled the one obtained in the shiverer (one peak in the 0.88 m region). When considering myelin basic protein (MBP) content in the normal developing animal, it was minimum in fraction A, mainly found in the B myelin peak, but also present in the light fraction (C + D). In young animals this latter peak was prominent, in contrast to the adult. In the Trembler mutant, the profile was close to normal; in the quaking, MBP was mainly found in peak A (0.85 m), and in the Jimpy MBP was very low and nearly constant throughout the gradient (with faint quantities in light C + D fraction). In mid, the content was very low, with a peak in the 0.88 m region, in contrast with its shiverer allele, where MBP is hardly detected. More meaningful myelin studies can be carried out by using zonal centrifugation in continuous sucrose gradient, to determine the density of mutant myelin and the degree of myelin maturation in animals.