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Downs syndrome (DS) occurs due to an extra copy of chromosome 21. About 3% of cases of Downs syndrome occur due to Robertsonian translocation, most commonly t (14; 21), other types of translocations are very rare cause of the syndrome. A 10- year-old patient with mental retardation was admitted following road traffic accident. Patient had flabby muscles, had delayed mile stones, stunted growth for the age, slanting of eyes, flat nasal bridge, and ineligible speech. On cytogenetic analysis the patient had karyotype showing one normal chromosome 21 and one Robertsonian translocation t (21; 21). Parents and siblings of the patient were phenotypically normal. Robertsonian translocation t (21; 21), can occur by transmission from carrier parent, due to ovarian mosaicism for Robertsonian translocation or may appear de novo. In the present case as the parents had normal karyotype and siblings were phenotypically normal, Robertsonian translocation probably have arisen de novo. The present case was a case of Downs syndrome with Robertsonian translocation t (21;21) probably arising de novo.