The sequences encoding the full-length epidermal growth factor receptor (EGFR) were cloned from a cDNA library prepared from T3M4 cells. A transition (G to A) was identified at codon 497 of EGFR cDNA, resulting in the substitution of a lysine for an arginine. The same substitution was identified by sequencing cDNA derived from 3 of 7 additional human pancreatic cancer cell lines, one endometrial cancer cell line, and one lung cancer cell line, but not in A431 cells. Variant EGFR was always co-expressed with wild-type EGFR. Both sequences were present in genomic DNA from two cell lines expressing the variant receptor and in DNA from normal (3 of 7 individuals) human lymphocytes. These findings indicate that there are two alleles in this region of the EGFR gene, and that expression of variant EGFR is a common occurrence in normal and cancerous cells.