This study addresses the contribution of an Sp1 response element in the proximal promoter of the transglutaminase 1 gene to transcription in normal epidermis and in a case of lamellar ichthyosis lacking transglutaminase 1 activity. The latter exhibited an Sp1 promoter mutation previously hypothesized to suppress transcription. In this study, several experiments indicated that the native Sp1 response element was functional, but it had only a small influence on transcription, and the previously observed mutation had no effect. These experiments involved mobility shift assays and transfections of promoter constructs in which the Sp1 site was mutated or lacking altogether. In addition the proximal 1.6 kb of the promoter from the affected individual was as active in transfections as the promoter from unaffected individuals. A search for sequence alterations in mRNA transcribed in keratinocytes from the patient revealed a novel single base mutation in codon 661 of the transglutaminase coding region predicted to result in premature termination of protein translation. The presence of this mutation in parental genomic DNA was confirmed by restriction digestion. Thus the lamellar ichthyosis phenotype in this case is likely attributable to a novel non-sense mutation in the coding region leading to reduced transglutaminase 1 mRNA levels rather than mutation of the Sp1 site.