Resolution of breakpoints in a complex rearrangement by use of multiple staining techniques: confirmation of suspected 12p12.3 intraband by deletion dosage effect of LDHB.

A complex de novo translocation was found in leukocytes and fibroblasts from a boy with mental retardation and minor abnormalities. The 45,XY chromosome constitution found in all cells was initially interpreted from routine G- and Q-banding techniques as a balanced translocation of part of the short arm of 12 to the short arm of 15 and of the long arm of 21 to the short arm of 12. With additional staining techniques and use of prometaphase chromosome preparations, it was determined that the distal portion of band 12.3 of the short arm of chromosome 12 was missing from the 12/15 and the 12/21 translocation chromosomes. This interpretation was confirmed by a decreased concentration of the LDH-B subunit in lactate dehydrogenase isozymes of the patient's fibroblasts, consistent with his being hemizygous for the LDHB locus.