The lens-specific reglatory element of the δ1-crystallin enhancer lies within the core segment (Goto et al., (1990) Mol. Cell. Biol. 10, 935-964). The element was allocated within the 55 bp long HN fragment of the core. Block-wise base substitutions were introduced to the 55 bp and their effect on the enhancer activity of the multimers in lens cells was examined. By base sequence alteration of either of the contiguous blocks 5 and 6, with their original sequence of TTGCT and C-ACCT, respectively, enhancer activity was totally lost. A lens nuclear factor δEF1 was found which bound specifically to the base sequences defined by the blocks. DNA binding activity very similar to δEF1 was also found in extracts of tissues other than lens, suggesting that δEFI participates in lens-specific regulation through tissue-dependent modification or interaction with other factors.