We report on monozygotic (MZ) twins with a de novo mos 46,XX,der(15)t(11;15)(p12;p11.2)/46,XX karyotype varying in different tissues. The clinical presentation and findings at the cytogenetic level are described. One of the infants had definite minor anomalies at birth, also found in other cases of trisomy of 11p resembling the Beckwith–Wiedemann syndrome. Theoretical backgrounds regarding the string of events leading to the cytogenetic findings in these twins and the various factors that might have contributed to the dissimilarities in phenotype between these twins are discussed. © 2003 Wiley-Liss, Inc.