Chromosome replication pattern was reevaluated in an unbalanced X/13 translocation carrier, 46, X, der (X), t(X;13)(q27;q12), using the BudR-acridine orange technique. The translocated chromosome, Xpter→q27::13q12→13qter, was late replicating in all analyzed cells. The autosomal segment showed a replication pattern comparable to the two normal, and presumably genetically active, chromosome 13’s, with the exception of band 13q22, which was consistently late replicating. Measurements of red-cell levels of esterase D (ESD) (a marker assigned to 13ql4) showed a 50% increase in ESD activity, compared to that in normal controls and parents of the patient. This suggests noninactivation of the ESD locus on the der(X) chromosome, a finding consistent with the late-replication