Fluorescence in situ hybridization detection of the monoallelic deletion of 17p13.1 in patient 28a. Two lymphocytes from this patient’s blood smear are shown, each with only one detectable signal for 17p13.1. This same pattern was observed in 35% of the blood leukocytes and constituted the diagnosis of monoallelic deletion of 17p13.1 (original magnification × 1,000).