The study of gene/dosage effect may be essential in tracing the pathogenetic steps which lead from an unbalanced chromosome anomaly to a pathological phenotype. We present a newborn with a clinical and pathological picture compatible with a diagnosis of Patau Syndrome. A chromosome analysis confirmed the diagnosis showing the presence of trisomy 13. On chromosome 13 the enzyme Esterase D (ESD) is mapped, and we demonstrated the gene/dosage effect for ESD in the erythrocytes of our patient.