T18d of BALB/c mice is a member of the Tla category of class I genes of the major histocompatibility complex of the mouse and is highly restricted in expression. Deletion analysis implies that an element essential to T18d expression resides within the region -4 to +54. The homologous region of T3d, a Tla gene which normally is not expressed in BALB/c mice, also has promoter activity. Thus the expressibility of T18d vs T3d is unlikely to be due to sequence differences in this region. A DNA-binding protein, factor VI, was found to bind to the region -33 to +54. DNase I footprinting analysis indicated that the DNA fragment 5'-ACTATAGTTTCACTTTTT-3' (+3 to +20) was protected by factor VI. This region includes the interferon response sequence (IRS). Homologous DNA segments of other class I genes, Ld and Dd, competed for factor VI in DNA-protein binding assay with lower affinity as compared with T18d. In mutation analysis, the 3' portion of the IRS is more important than the 5' portion with respect to binding affinity of factor VI and to transcriptional activity in transfected cells. This result signifies a role of IRS in T18d transcription and suggests that the mechanism of T18d transcription might be unusual.