The function of small GTPases is fine-tuned by a complex network of regulatory proteins such as GTPase-activating proteins. The C1 gene at Xq28 encodes a protein assumed to function as a Rho GTPase-activating protein (rhoGAP). Characterization of the molecular defect causing X-linked nephrogenic diabetes insipidus (NDI) in a patient revealed a submicroscopic deletion of a 21.5-kb genomic fragment encompassing the entire arginine-vasopressin V2 receptor gene (AVPR2) and most of the C1 gene locus. In the absence of detailed information about the physiological relevance and specific functions of rhoGAP C1, a thorough clinical and laboratory investigation of the patient was performed. Besides clearly defined NDI symptoms caused by deletion of the AVPR2 gene, no major morphological abnormalities as determined by physical examination, radiography, ultrasound, and computed tomographic scan were detected. Extensive analysis of blood chemical, enzyme, and hormone values over a period of 16 years showed no deviations from normal ranges. On the basis of our observations, the rhoGAP C1 protein is not essential for normal development in the human. Because of a predominant expression pattern of the C1 gene in hematopoietic cells, we focused on immunologic and hematologic laboratory parameters of the affected boy and the mother who was found to be heterozygous. Differential white cell counts, including lymphocyte typing, determination of lymphokines, cytokines, and immunoglobulins, as well as numerous leukocyte function tests, showed no pathological findings. Therefore, we postulate that the loss of rhoGAP C1 function is most likely compensated by other members of the GAP family.