We studied a child with apparent monosomy of chromosome 21. Cytogenetic, FISH and microsatellite analyses revealed a 45,X,-21,+der(X)t(X;21)(q25;q21.1) karyotype resulting from a de novo, unbalanced, X;21 non-reciprocal translocation of paternal origin, with partial monosomy of chromosomes 21 and X. An extreme, skewed X-inactivation pattern of the der(X) chromosome was demonstrated. Skewed inactivation probably accounted for a mild phenotype with respect to Xq25-->qter deletion while propagation of inactivation to the adjacent 21q region may account for mild clinical features associated to distal 21q monosomy.