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Argininosuccinicaciduria is a rare genetic error of the Krebs-Henseleit urea cycle due to the absence or deficiency of the enzyme argininosuccinate lyase and is associated with mental retardation, seizures, ataxia, and hepatomegaly. Hyperammonemia may or may not be present, but blood urea levels are always within normal limits. Abnormally fragile hair has been detected in about half of the reported cases, and in some, a morphologic hair alteration termed trichorrhexis nodosa has been observed. 1 The nature of the hair defect has not been extensively studied, and tensile strength and other physical properties have been measured in only a single case, to our knowledge. 2 Amino acid analysis of acid hydrolysates of our patient's hair revealed a reduction in cystine content to 50% of the control level, concomitant with a similar degree of reduction of values for tensile strength and Young's modulus. No relationship between the urea cycle block, the