Somatic chromosomal mosaicism arising from post-zygotic errors is known to cause 
several well-defined genetic syndromes as well as contribute to phenotypic 
variation in diseases. However, somatic mosaicism is often under-diagnosed due 
to challenges in detection. We evaluated 10,362 patients with a custom-designed, 
exon-targeted whole-genome oligonucleotide array and detected somatic mosaicism 
in a total of 57 cases (0.55%). The mosaicism was characterized and confirmed by 
fluorescence in situ hybridization (FISH) and/or chromosome analysis. Different 
categories of abnormal cell lines were detected: (1) aneuploidy, including sex 
chromosome abnormalities and isochromosomes (22 cases), (2) ring or marker 
chromosomes (12 cases), (3) single deletion/duplication copy number variations 
(CNVs) (11 cases), (4) multiple deletion/duplication CNVs (5 cases), (5) exonic 
CNVs (4 cases), and (6) unbalanced translocations (3 cases). Levels of mosaicism 
calculated based on the array data were in good concordance with those observed 
by FISH (10-93%). Of the 14 cases evaluated concurrently by chromosome analysis, 
mosaicism was detected solely by the array in 4 cases (29%). In summary, our 
exon-targeted array further expands the diagnostic capability of high-resolution 
array comparative genomic hybridization in detecting mosaicism for cytogenetic 
abnormalities as well as small CNVs in disease-causing genes.