In duck the single copy lactate dehydrogenase B (LDH-B) gene also encodes an abundant lens protein, epsilon-crystallin. The LDH-B/epsilon-crystallin gene consists of eight exons, of which the first is non-coding. The promoter region lacks a TATA box, is very GC-rich and contains multiple consensus Sp1 binding sites. The gene has two discrete transcription start sites located 28 base-pairs apart. Both sites are used about equally in heart tissue, while transcription from the downstream start site predominates in the lens. For maximal promoter activity in lens or heart, sequences from the first intron are required. The enhancer(s) in this intron is promoter specific as it could not activate the tk promoter. Studies at the RNA level show that the overexpression of the LDH-B/epsilon-crystallin gene in the lens is regulated at the transcriptional level, yet no tissue-specific regulatory elements could be detected in a region spanning from -1.9 kb (1 kb = 10(3) bases or base-pairs) up to the translation initiation site in the second exon. The basis for the differential expression of the LDH-B/epsilon-crystallin gene in duck heart and lens is the usage of the downstream transcription initiation site. However, our results do not allow a distinction between activation of the downstream transcription start site in the lens or repression of the use of this site in heart.