A mouse mutant with sarcosinemia was found by screening the progeny of ethylnitrosourea-mutagenlzed mice for aminoacidurias. Paper chromatography, column chromatography, and gas chromatography-mass spectrometry identified high levels of sarcosine in the urine of the mutant mice. While sarcosine cannot be detected in the urine or plasma of normal mice, the urinary sarcosine level of 102 ± 58 mmol per g of creatinine in the mutant mice was at the upper range of the urinary levels (1.545 mmol of sarcosine per g of creatinine) observed in humans with sarcosinemia. Similarly, the plasma sarcosine level of 785 ± 153 pzmol/liter in the sarcosinemic mice was at the upper range of the plasma sarcosine levels (53-760 pzmol/liter) observed in affected humans. Sarcosine dehydrogenase [sarcosine:(acceptor) oxidoreductase (demethylating), EC 1.5.99.1] activity was deficient in sarcosinemic mice. The sarcosinuria phenotype in these mice was inherited as an autosomal recessive trait. This mouse mutant provides a useful genetic model for human sarcosinemia and for development of therapeutic approaches for genetic disease.Genetically modified mice are increasingly being used as models for diseases and for development of approaches for treatments such as gene therapy. Although some useful mouse strains arose spontaneously, two basic approaches have been developed to produce mouse mutants. When the gene is known beforehand, site-directed mutation induced by homologous recombination in embryo stem cells is a powerful approach for creating mouse mutants (1). When only the phenotype is known, germline mutagenesis followed by backcrossing and phenotypic screening has been successful. The latter approach utilizing ethylnitrosourea (ENU) mutagenesis has been used to develop hyperphenylalaninemic mice strains, deficient in phenylalanine hydroxylase or GTP cyclohydrolase activities (2, 3), and muscular dystrophic mice with defective dystrophin (4).Mouse models exist for a few of the inborn errors of metabolism (IEM) (5). Many genes related to IEM in humans have been defined and could be targeted for disruption in mice by homologous recombination. Others, however, have not been identified to date. In many IEM, such as cystinosis or Hartnup disease, the defective protein has yet to be identified. Furthermore, several disorders with poorly defined metabolic defects such as lactic acidosis or 3-methylglutaconic aciduria also have poorly defined enzymatic deficiencies (6). Genetic mouse models for these IEM should be useful for assigning the chromosomal location and for eventual isolation of the relevant genes (7).This report describes a mouse mutant with sarcosinemia found through metabolic screening of progeny of ENUmutagenized mice. The breeding scheme and ENU mutagenesis protocol were identical to that used for creating the hyperphenylalaninemic mutants (8). The phenotype testing was based on the metabolic screens that have been developed for newborn screening (9, 10). Of 135 pedigrees evaluated, one mutant strain was found to have ...