AbstractA case of partial 3p trisomy is reported here. A review of published cases (8 ♂, 2 ♀, 7 families) shows a characteristic pattern of anomalies, constituting one more syndrome of multiple congenital anomaly and mental retardation (MCA/MR) characterized by microcephaly, brachycephaly, frontal bossing, temporal indentation, square face, hypertelorism or telecanthus, epicanthus, short nose with a large tip, prominent cheeks, long and protruding philtrum, large and downturned mouth, protruding mid‐upper lip, micro‐ or retrognathia, short neck, congenital heart defects, gastrointestinal malformation, penile hypoplasia, neuromotor or mental retardation, and predominance of whorls on digits.The proposita had a 46,XX,der(11),t(3;11)(p21;q25) karyotype. The mother was a carrier of a de novo 3;11 balanced translocation. Chromosome mosaicism was detected in a female sibling of the proposita: 46% of her cells were 46,XX and 54% had a 46,XX,t(3;20)(p21;q13) karyotype ‐ ie, a de novo 3;20 balanced translocation. We discuss the origin of this mosaicism and the possible meaning of the breaks involving the same region of chromosome 3 (region 3p21) in the members of the proposita