An intraventricular fetus-in-fetu, a malformed monochorionic diamniotic twin, was identified in a 1-year-old girl with motor delay and enlarged head circumference (Figure 1). After surgical removal, whole-genome sequencing revealed identical… Click to show full abstract
An intraventricular fetus-in-fetu, a malformed monochorionic diamniotic twin, was identified in a 1-year-old girl with motor delay and enlarged head circumference (Figure 1). After surgical removal, whole-genome sequencing revealed identical single-nucleotide variants in the host child and fetus-in-fetu, with extensive de novo copy number gains in the fetus-in-fetu (Figure 2, eMethods, links.lww.com/WNL/C529), suggesting the significance of copy number variation during embryogenesis.
               
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