Specific pairs of heterotypic keratin chains are coexpressed during the various stages of epithelial differentiation. This pattern of expression of cytokeratin pairs implies the existence of highly coordinated control of transcription. The gene for cytokeratin 3 (KRT3), a member of the type II subfamily, is expressed together with KRT12  during 'corneal-type' differentiation (Schermer et al., 1986).

The KRT3 gene is approximately 6.5 kb long and encodes one of the largest cytokeratin proteins (64 kD). By fluorescence in situ hybridization, Raimondi et al. (1994) mapped the KRT3 gene to 12q12-q13. All type II keratin genes mapped to that time had been assigned to chromosome 12 where they appeared to be organized into one homotypic cluster.