The Sam68 mRNA is known to be widely expressed [38], whereas its pattern of protein expression in vivo remains to be defined. Sections of paraffin-embedded wild-type E14.5 and E16.5 embryonic mice were immunostained with the well-characterized AD1 anti-Sam68 antibody, raised in rabbits against an immunizing peptide that corresponds to amino acids 330 to 348 of the mouse Sam68 protein [46]. Sam68 immunoreactivity was observed in the skeleton and soft tissues of developing wild-type mice at E14.5 and E16.5 (Figure 1). Intense staining was seen in the nucleus of cells in the developing brain, heart, and small intestine (Figure 1B), as well as in chondrocytes in the nasal septum and the glandular tissue adjacent to the nasal cartilage (Figure 1C, panels A–D), in the vertebra and intervertebral discs (panels E–H) and in the epiphysis (panels I–K) and metaphysis (panel L) of long bones. Nuclear staining was observed in proliferating chondrocytes and hypertrophic chondrocytes (Figure 1C, panel H), in osteoblasts (panel L; Dataset S1), and in osteoclasts (Dataset S1). No staining was observed with preimmune serum or when the antiserum was preadsorbed with the immunizing peptide (Figure 1C, panels B, F, and J). Taken together, these data demonstrate that Sam68 protein is selectively expressed in the developing mouse embryo, with particularly elevated expression in cartilage and bone.