Extract: A dominantly inherited trait, expressed as ‘hyperdibasicaminoaciduria’, has been identified in 13 of 33 members in a French Canadian pedigree (fig. 1). The female proband was 18 months old when first identified. The medical problems which brought her to our attention were her small stature and symptoms compatible with a mild malabsorption syndrome. Neither the small stature nor the intestinal complaints could be linked confidently with the appearance of the inherited trait; it is more reasonable to assume that the incidental appearance of a medical problem in the proband brought the otherwise benign trait to attention. The trait was characterized by abnormally high urinary excretion rates for the diaminomono-carboxylic (‘dibasic’) compounds, lysine, ornithine, and arginine; cystine, which is excreted in abnormal amounts along with the dibasic amino acids in classical cystinuria, was excreted normally in this instance (table I and figs. 2A and 2B). This constitutes the distinctive characteristic of the trait. It allows one to discriminate the carriers of the trait, who presumably are heterozygotes, from carriers of the various cystinuric genotypes. The trait is expressed consistently with regard to interindividual variation (table I) and intraindividual variation (table II). The plasma concentrations of the affected amino acids were normal in carriers of the trait, while the endogenous renal clearance rates were modestly elevated (table III). A defect in net tubular absorption of the relevant amino acids thus existed. The mutant transport trait was apparently also expressed in the intestine. This is assumed because net accumulation of lysine in plasma after loading by mouth was less in carriers of the trait than in normal subjects (fig. 3 and table IV); in contrast, in the single patient tested, intestinal absorption of cystine was not impaired. The characteristics of the trait suggest that the diaminomonocarboxylic amino acids share a transport system in kidney and intestine which excludes cystine. Speculation: The discovery of a mutant trait, in which transport of diaminomonocarboxylic acids alone is impaired, illustrates the specificity of membrane transport systems for amino acids. In this instance, the belief that cystine is not transported on the ‘dibasic’ system is confirmed. Thus, an interpretation of the physical and chemical basis for classical cystinuria, where cystine and ‘dibasic’ amino acids are affected together, still remains a major challenge to the investigator.