Myelin consists of a continuum of particles of different density with varying lipid composition: major differences are found between normal mice and quaking mutants.

Mouse brain myelin consists of a continuum of particles of different densities, as shown by sucrose density gradient centrifugation. In normal animals most of the material (65 per cent) is concentrated between 0.6 and 0.7 M sucrose (the maximum being found at 0.66 M sucrose, corresponding to 23 per cent). The density differences among various myelin fractions are related to their protein/lipid ratios, as lighter fractions contain less protein and more lipid. Lipid analysis shows a decrease in the amount of every lipid from the lightest to the heaviest fraction: the light fraction is richer in phosphatidyl-ethanolamine, phosphatidyl-serine and cerebrosides. The distribution is highly abnormal in purified myelin from Quaking mutant ; very low quantities of myelin with normal density are found, but unexpected large amount of high density particles are found, possibly related to a "pre-myelin" material (oligodendrogial) processes which are not maturing into normal myelin).