As a result of the first round of investigations (17 DNA STRs generated by using Powerplex 16 and AmpFlSTR Identifiler), 2 inconsistencies at loci Penta D and D215S11 (both located on chromosome 21) were observed. Since 2 exclusions were not considered sufficient to pronounce non-paternity, 11 additional STRs, using the Humantype Chimera kit, were analyzed; however, this step did not reveal any additional genetic inconsistency but did show that at locus D21S2055 the mother and child carried identical phenotypes as they did at loci D21S11 and Penta D, respectively. The authors decided to study Y chromosomal markers, which revealed one further incompatibility between the father and the child at locus DYS389 II. This third incompatibility led to performing restriction fragment length polymorphism typing using seven single-locus probes (MS1, MS31, MS43A, G3, YNH24, MS205, LH1), all of which generated results consistent with the paternity of the alleged father. These results and the identical chromosome 21 phenotypes between child and mother suggested a maternal uniparental disomy; therefore, additional typing of chromosome 21 markers, routinely used in clinical genetics, was performed. All chromosome 21 alleles of the child were exclusively of maternal origin. Uniparental disomy is a rare phenomenon that occurs when a person receives two copies of a chromosome, or a part of a chromosome, from one parent and no copy from the other parent, respectively. To the knowledge of the authors, the exact frequency of this phenomenon, especially of this type of uniparent disomy at chromosome 21, is unknown. 1 table, 1 figure, and 17 references