A 10-year-old girl and a 10-month-old girl, both with ambiguous genitalia, were found to have 45,X/46,X,mar and 45,X/46,X,r(?) mosaicism. The marker chromosomes in both girls were very small. Polymerase chain reaction, with synthetic oligonucleotide primers from Y-specific DNA sequences pY-80 and pY53.3 containing the sex-determining region Y(SRY), proved the marker chromosomes to contain the Y short arm material. In situ hybridization with probe pY-80 confirmed that the marker chromosomes included the Y short arms. These findings, together with ambiguous genitalia in the girls, indicate that the marker chromosomes include the testis-determining factor gene.