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. 2011 Jul;4(4):457–467. doi: 10.1242/dmm.007245

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© 2011. Published by The Company of Biologists Ltd

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial Share Alike License (http://creativecommons.org/licenses/by-nc-sa/3.0), which permits unrestricted non-commercial use, distribution and reproduction in any medium provided that the original work is properly cited and all further distributions of the work or adaptation are subject to the same Creative Commons License terms.

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Fig. 2. — The threshold hypothesis of SMA. There seems to be a differential susceptibility of cell types and tissues to SMN reduction. Motor neurons are most severely affected by SMN depletion and are thus at one end of a vulnerability-resistance spectrum. As protein levels are further reduced, additional tissues such as bone, heart and sensory neurons become affected, while other cells and tissues remain unaffected. For example, at approximately 10–20% of normal SMN levels, as seen in Type I SMA patients, motor neurons are severely affected, the heart and brain CNS show partial involvement, whereas many organs remain relatively unaffected. See Box 1 for details.