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Editor—Approximately 33 cases of trisomy 20p have been reported.1-10 Most cases are the product of reciprocal translocations with a few cases arising from inversions. A trisomy 20p syndrome has been difficult to delineate as many cases involve only partial trisomy, often in the presence of partial monosomy of the partner chromosome. We describe a case of pure trisomy 20p arising from de novo isochromosome formation associated with non-reciprocal translocation. This type of chromosome rearrangement is very rare and to date has been described only for isochromosome formation of chromosomes 4p, 5p, 7p, 9p, 10p, and 12p.11 12 13 The rarity of these cases is the result of selection bias as only those partial trisomies compatible with life will be ascertained. Our case, involving duplication 20p with no other chromosomal imbalance, is important to help delineate this syndrome, which is not yet clearly defined. The boy, now aged 19, had dysmorphic features, mild to moderate learning difficulties, osteopenia, and renal abnormalities. He was the second of two children of unrelated, normal Indian parents aged 35 years (mother) and 37 years (father). His facial features included epicanthic folds and anteverted, flared nostrils as a baby. As a child and adult he had a low anterior hairline, coarse hair, and laterally arched eyebrows. He had a very prominent, large nose, noticeable as a young child, with a convex nasal bridge and anteverted nostrils, present to a much lesser degree in his father. He had short, upward slanting palpebral fissures, a featureless philtrum, thin vermilion border of the upper lip, and a prominent lower lip. His ears were large and low set with a bilaterally prominent antihelix. He had a high arched palate, moderate micrognathia, and dolichocephalic skull (figs 1 and 2). Figure 1 (A, B) Front and lateral view of …