We have cloned and sequenced a full length cDNA for the light subunit of human liver gamma-glutamylcysteine synthetase (GCS1). The GCS1 cDNA consists of 1628 bp, containing an open reading frame of 822 bp which encodes a protein of 274 amino acids having a calculated M(r) of 30,729 Daltons. Human liver GCS1 shares 91% and 96% sequence homology at the nucleotide and amino acid levels, respectively, with its rat counterpart (Huang, Anderson and Meister, J. Biol. Chem. 268:20578-20583, 1993). Transcripts of 1.4 and 4.1 kb in length were detected by Northern blot analysis of mRNA isolated from each of sixteen different human tissues. Relative expression of the two GCS1 RNA transcripts was highly tissue-dependent. High steady-state levels of the smaller transcript were detected in colon, while very high levels of both transcripts were found in skeletal muscle. Expression of mRNA transcripts for the GCS heavy subunit were likewise tissue-dependent, and did not necessarily correlate with the level of GCS1 transcripts in any tissue.