We report on a reciprocal translocation t(X;16)(q28;p12) detected in a newborn girl with clinical manifestations of partial trisomy 16p. A balanced translocation was found in the mother and in the maternal grandmother. Replication studies on lymphocytes and fibroblasts showed nonrandom X-inactivation in both the patient and her mother. In the mother, the derivative X (der(X)) was active, whereas the normal X was late replicating. In contrast, in the patient the der(X) was late replicating, and there was no spreading of X-inactivation onto the autosomal segment, thus giving an explanation for the full clinical picture of partial trisomy 16p.