Bone marrow somatic mutation frequency, as mea­sured by the glycophorin A (GPA) assay in circulating red blood cells, has been found to be increased in some hered­itable disease syndromes associated with constitutional malformation, mental retardation, cancer susceptibility and premature aging. Increased spontaneous GPA mutation frequency has been shown to be diagnostic for ataxia tel­angiectasia (AT) and Fanconi’s anemia (FA). We have applied the GPA assay to a 11 pediatric patients referred for cytogenetic analysis for a variety of reasons, including developmental delay and dysmorphology. There was no evidence of any cytogenetic abnormality in these patients, so we investigated the possible role of constitutive genom­ic instability expressed as a high frequency of bone mar­row somatic mutation. These patients had normal overall levels of GPA somatic mutation when compared to new­born and pediatric controls. When phenotypic subsets of mutants were considered, however, the patients had de­creased levels of simple allele loss mutation and increased levels of allele loss and duplication. Such a shift in types of mutation without an accompanying increase in overall mutation is difficult to understand, but may be related to inherent cytotoxic susceptibility and cellular inviability in this heterogeneous patient population.