In the early 1950's, Harris, Mittwoch, Robson, and Warren (1, 2) investigated the mode of inheritance of cystinuria in 27 families by using quantitative determinations of cystine and dibasic amino acids as the genetic marker. Homozygotes were identified by the formation of urinary tract calculi composed of cystine and by gross hyperexcretion of cystine, lysine, arginine, and ornithine. Investigation of known heterozygotes (parents and children of affected subjects) revealed distinct phenotypic heterogeneity and identified two types of families. In one, comprising about two-thirds of the pedigrees studied, heterozygotes uniformly excreted normal quantities of cystine and dibasic amino acids, and genetic analysis was compatible with autosomal recessive inheritance. In the second, smaller group of pedigrees, an intermediate phenotype was found. All heterozygotes tested excreted moderate excesses of cystine and lysine.
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