Supernumerary Marker Chromosome in a Child with Microcephaly and Mental Retardation.

A de novo supernumerary marker chromosome (SMC) was identified in a 13- month-old girl who presented with microcephaly and mild mental retardation. On further characterization by oligo-nucleotide array-comparative genomic hybridization [array-CGH], the SMC was confirmed to be 18p.