The combinatory phenotype of thrombocytopenia and developmental delay has been described for two genetic conditions: a chromosome 11q deletion that is referred to as Jacobsen syndrome, and a 21q22 microdeletion syndrome. Herein, we report a young girl who presented with persistent macrothrombocytopenia and a developmental delay. Whole exome sequencing revealed a de novo amino acid substitution in CDC42, a critical regulator of the cytoskeleton. Our observation recapitulates observations in mice lacking Cdc42. We suggest that this CDC42 mutation may represent yet another mechanism leading to the combinatory phenotype of persistent macrothrombocytopenia and developmental delay.
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