Abstract
LINE-1 (L1) has achieved its status as a middle repetitive DNA family in mammalian genomes by duplicative transposition. Although transposition may occur in any cell type, expression and transposition of a full-length functional element in the germ line are necessary for evolutionarily significant propagation of L1. An immunohistochemical analysis of adult mouse ovaries and mouse postimplantation embryos revealed expression of L1 open reading frame 1 in the germ line as well as in steroidogenic tissues. These results demonstrate that L1 expression is controlled by a tightly regulated temporal and spatial program of events during development and imply that multiple loci of L1 in the mouse genome are active for expression.