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The gene lin-28 was originally identified through a mutant of the nematode Caenorhabditis elegans displaying defects in developmental timing. It is expressed stage-specifically in tissues throughout the animal and is required for cell fates to be expressed at the appropriate stage of larval development. lin-28 encodes a cytoplasmic protein with a unique pairing of RNA-binding motifs. Diverse animals possess Lin-28 homologues and mouse Lin-28 is expressed in embryos, embryonic stem cells and embryonal carcinoma cells, but not in some differentiated cell types. To assess whether mammalian Lin-28 may function as a developmental timing regulator, we examined adult and embryonic tissues of the mouse for its expression. We observed Lin-28 protein in many diverse tissues of the embryo through the period of organogenesis and that it persists in some tissues in the adult. In addition to an overall down …