Microdeletions

Several genetic syndromes have now been identified by
molecular cytogenetic techniques as being due to chromosomal
deletions too small to be seen by conventional analysis. These
are termed submicroscopic deletions or microdeletions
and probably affect less than 4000 kilobases of DNA. A
microdeletion may involve a single gene, or extend over several
genes. The term contiguous gene syndrome is applied when
several genes are affected, and in these disorders the features
present may be determined by the extent of the deletion. The
chromosomal location of a microdeletion may be initially
identified by the presence of a larger visible cytogenetic
deletion in a proportion of cases, as in Prader–Willi and
Angelman syndrome, or by finding a chromosomal
translocation in an affected individual, as occured in William
syndrome.