Abstract
BACKGROUND Down syndrome, or trisomy 21, is one of the most common chromosomal disorders associated with intellectual disability. Prenatal screening is a proactive approach to identify fetuses with common chromosomal abnormalities. Mosaicism is one of the causes of false-negative results in prenatal screening for Down syndrome. CASE REPORT We present a case of a newborn with tissue-specific and intercellular mosaicism for trisomy 21. Postnatal karyotyping of phytohemagglutinin-stimulated lymphocytes confirmed trisomy 21 in all analyzed metaphases. However, fluorescence in situ hybridization on unstimulated lymphocyte nuclei and buccal smears revealed mosaicism, with approximately 20% of cells from both tissues displaying disomy for chromosome 21. The intercellular karyotypic discordance observed in this case shows that mosaicism can be more complex than what is detectable by conventional karyotyping, and that it can be related to the biological particularities of the analyzed cells and restriction of a widely used cell division stimulator, rather than to technical limitation. CONCLUSIONS Our findings underscore the critical importance of combining karyotyping with fluorescence in situ hybridization on unstimulated lymphocytes and buccal smear cells to improve the accuracy of cytogenetic diagnosis in newborns with suspected Down syndrome. Such in-depth cytogenetic analyses provide essential information for genetic counseling, research on genotype-phenotype correlations, and evaluation of age-acquired mosaicism and its association with age-related comorbidities in patients with trisomy 21. Comprehensive identification of the biological causes underlying false-negative results in prenatal screening can help overcome the limitations of current technological platforms and support the refinement of diagnostic algorithms.