A series of tandem duplications of an ancestral cathepsin L gene has given rise to a family of
eight placenta-specific cathepsins in mice. These genes are differentially regulated both
spatially and temporally and thus each can perform unique placental functions. Analysis of
the function and expression of these genes is yielding new insights into gene regulation and
proteolytic processes in placenta, and may dissect critical placental roles of the single
human functional ortholog, cathepsin L.