Background: Small supernumerary marker chromosomes (sSMCs) are extra, structurally abnormal chromosomes with diverse clinical consequences.This study aimed to determine the frequency, molecular characteristics, and real-world diagnostic outcomes of sSMCs identified at a single tertiary center in Korea over a 17-year period.Methods: We retrospectively reviewed 3,924 patients who underwent conventional karyotyping for constitutional abnormalities at Pusan National University Yangsan Hospital between December 2008 and September 2025.For the 16 patients identified with sSMCs, medical records were examined, and results from fluorescence in situ hybridization (FISH), chromosomal microarray (CMA), and polymerase chain reaction (PCR) for the SRY gene were analyzed.Results: The prevalence of sSMCs was 0.41% (16/3,924).Among these cases, 43.8% (7/16) were mosaic, and 43.8% (7/16) occurred in patients with Turner syndrome (TS).The chromosomal origin was determined in 62.5% (10/16), with the Y chromosome (5/16, 31.2%) and chromosome 15 (4/16, 25%) being the most common.All sSMC derived from chromosome 15 (sSMC(15)) cases showed pathogenic copy-number gains involving the 15q11-q13 region, associated with developmental delay.Six characterized sSMCs in patients with TS originated from sex chromosomes (five sSMC(Y) and one sSMC(X)), and one patient with an sSMC(Y) developed dysgerminoma.Six sSMCs (37.5%) remained uncharacterized due to normal CMA results or incomplete molecular workups.Conclusions: This study reports a 0.41% prevalence of sSMCs, consistent with previous findings, and highlights the clinical significance of sSMC(15) and sSMC(Y).Our results underscore persistent diagnostic challenges and emphasize the need for an integrated approach combining karyotyping, molecular techniques, and parental studies to support accurate prognostication and genetic counseling.
Ji et al. (Fri,) studied this question.