The gene encoding sericin 1 (Set1) of silkworm (Bombyx mori) is specifically expressed in the middle silk gland cells. To identify element involved in this transcription-dependent spatial restriction, truncation of the 5' terminal from the sericin 1 (Ser1) promoter is studied in vivo. A 209 by DNA sequence upstream of the transcriptional start site (-586 to -378) is found to be responsible for promoting tissue-specific transcription. Analysis of this 209 by region by overlapping deletion studies showed that a 25 by region (-500 to -476) suppresses the ectopic expression of the Ser1 promoter. An unknown factor abundant in fat body nuclear extracts is shown to bind to this 25 by fragment. These results suggest that this 25 by region and the unknown factor are necessary for determining the tissue-specificity of the Set1 promoter. (c) 2006 Elsevier Inc. All rights reserved.