skeleton
• penetrance about 50%
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• rarely, only the ventral portion of the first rib is present
• sometimes a gap exists between the dorsal and ventral portions of the first rib
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• absence or shortening of first rib
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• bifurcation of the second rib so that the upper branch articulates with the top of the sternum
• bifurcation occurs at the point bone-cartilage junction in newborns
• first detectable around E11.5
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• altered lateral processes on T1
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• T1 to C7 transformation the most prevalent vertebral abnormality
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• C7 to C6 transformation
• tuberculum anterior of C6 either shifted to C7 (C6 to C5 transformation) or duplicated there
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nervous system
• path of first intercostals nerve altered due to the abnormal rib structures
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