Lec1 CHO cell mutants lack N-acetylglucosaminyltransferase I (GlcNAc-TI) activity and do not synthesize complex or hybrid N-glycans. The origins of six independent lec1 mutations are shown to reside in the coding region of the Mgat1 gene, proving that GlcNAc-TI is mutated in Lec1 mutants. One mutant has Mgat1 gene transcripts of reduced size, whereas the others possess transcripts of approximately normal size and amount containing a unique insertion or transition mutation that leads to a premature stop codon in the Mgat1 gene coding region. The lec1 mutation in the Lec3.2.8.1 mutant, a line used to generate minimally glycosylated membrane glycoproteins for X-ray crystallography, is a G insertion that leads to a nonsense codon after amino acid 391. The Pro-Lec1.3C line from the ATCC and in laboratory stocks, a line used widely for diverse purposes, possesses a C insertion in the Mgat1 gene coding exon, causing a frame shift and producing a stable, truncated approximately 24-kDa product. Mgat1 gene mutations were confirmed by sequencing genomic DNA PCR products. Mutant cDNAs were reverted by site-directed mutagenesis and shown to confer wild-type lectin binding and GlcNAc-TI activity on Lec1 transfectants. Surprisingly, three Mgat1 gene nucleotide changes previously reported in Pro-Lec1.3C cells (Puthalakath et al. [1996] J. Biol. Chem., 271, 27818-27822) were not detected in this study. These Lec1 mutants provide a novel cohort for investigating the effects on Golgi trafficking and kin recognition of deletion mutants of GlcNAc-TI expressed at endogenous rather than nonphysiological levels.