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Dear Editor, Trisomy 20p is a rare genetic disorder manifesting as intellectual disability, speech delay, specific facial features, and delayed motor milestones. Severity of the symptom depends on chromosome 20p duplication size; larger chromosomal duplications usually result in more serious symptoms [1]. Most previously reported cases involved partial trisomy 20p derived from a parental reciprocal translocation, chromosome inversion, or a small supernumerary marker chromosome (sSMC) [1-5]. However, only a few cases of pure trisomy 20p (involving whole short arm of chromosome 20) have been reported [1, 5-9]. We report a case of pure trisomy 20p arising from a de novo marker chromosome, 20p(47,XX,+mar), with a non-reciprocal translocation, which was characterized at the molecular level by comparative genomic hybridization (CGH). To the best of our knowledge, this is the first case of de novo pure trisomy 20p arising from a marker chromosome in Korea. In addition, we reviewed previously reported cases of trisomy 20p syndrome and compared them with the phenotype of our patient. We received informed consent from the patient for all the genetic testings. This study was approved by the Institution Review Board of Korea University Guro Hospital, Seoul, Korea (IRB-2019GR0429). The patient was referred at 30 months of age, in June 2017, to Korea University Guro Hospital, Seoul, Korea, for an evaluation of developmental and speech delay. She was the last of four children of unrelated healthy Korean parents aged 39 (mother) and 40 years (father), respectively. Physical examination showed facial asymmetry, micrognathia, strabismus, and a large ear; neither cardiac syndrome nor renal abnormalities were observed. The neurological examination revealed poor coordination in gross and fine motor skills. Brain magnetic resonance imaging did not reveal any structural abnormalities in the auditory system. In the developmental and speech delay evaluation, the scores were <0.1% of the scales defining moderate mental retardation in the Korean Bayley Scales of Infant and Toddler DevelopmentII and <1% of the scales indicating speech delay Sequenced Language Scale for Infants. However, the Childhood Autism Rating Scale score was 27 (cutoff: 30); thus, she was classified as a non-autistic child. The cytogenetic examination was carried out on peripheral blood of the patient and her parents. Chromosome harvesting and karyotyping were performed following a standard protocol, phytohemagglutinin-stimulated peripheral blood culture. The proband’s karyotype was defined as 47,XX,+mar in metaphase (Fig. 1A), while the mother and father had normal karyotypes of 46,XX and 46,XY, respectively. Genomic DNA was ob-

De Novo Pure Trisomy 20p: Report of a Novel Case of a Marker Chromosome and Literature Review