Besides genes and pseudogenes, there are other architectural features in the LCR22s that are noteworthy. As mentioned, LCR22-3a harbors an AT-rich palindrome that mediates the recurrent t(11;22) translocation and perhaps the 1.5-Mb deletion in VCFS/DGS patients (Fig. 1). The AT-rich sequence and surrounding sequences have remained uncloned and are missing from the human genome libraries. It is likely that they are absent because of the palindromic nature of the AT-rich stretch of sequence in humans. Interestingly, a similar pattern of AT-rich sequences, although non-palindromic, is present in LCR22-2 and LCR22-4. It is possible that in some humans, they could be palindromic in nature but have rearranged in bacteria during library formation, thereby deleting sequences. The presence of potential palindromic sequences in LCR22-2 and LCR22-4 is particularly interesting as recently palindromic sequences have been found near the hotspot for the common 17p11.2 deletion associated with SMS and its reciprocal duplication dup(17)(p11.2 p11.2) syndrome (72).
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