(A) A set of sequence fragments (e1–e4), each of which functions as an enhancer in a reporter gene assay. Fragment e2 is active in both Tissue A and Tissue B, identical to e1, but was identified more recently. e3 and e4 each have a subset of the function of e1 and e2, with distinct activity. (B) In this second set of sequence fragments, the more recently-identified e2 is longer than the original e1 enhancer, but lacks ectopic regulatory activity in Tissue C. Thus e2 is more properly the “true” enhancer than the shorter e1.