Distribution of the proportion of HbH in the peripheral blood in ATMDS compared with ATR-X syndrome. Results of supravital staining have been reported in 61 patients with ATMDS, and the number of inclusion-containing cells has been quantified in 54 patients. Of these, 17 (31%) patients have had 50% or more circulating erythrocytes containing HbH inclusions, 22 (44%) have had 10% to 50% of such cells, and 15 (28%) patients have had 10% or less (median, 30%). Only 8 (17%) of 48 quantified cases have had 25% or more HbH when measured by electrophoretic or chromatographic methods (data not shown), and in 10 patients the amount of HbH was below the level of detection despite the presence of small numbers of inclusions on supravital staining (mean, 15%). Several patients with ATR-X syndrome have no HbH, whereas all patients with ATMDS have some. This is because, by definition, patients with ATMDS must have some evidence of HbH, but ATR-X syndrome may be diagnosed from other characteristic nonthalassemic features of the multisystemic disorder in the presence of an ATRX gene mutation.