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Some nuclear receptors have defined natural ligands, such as steroid hormones, thyroid hormones, retinoids, or vitamin D, but others have no identified ligand and are called orphan receptors. The finding that diverse compounds act as ligands for nuclear receptors and that some receptors have no apparent ligand led to the hypothesis that ancestral nuclear receptors were constitutive transcription factors that independently evolved the ability to bind ligand. 6,7 However, a second hypothesis posits that ancestral receptors were ligand-dependent transcription factors that evolved specificity for different ligands by gene duplication, mutation, and functional divergence. There are several lines of evidence that favors the latter hypothesis for the evolution of ligand binding in the steroid receptor family. First, the primary, secondary, and tertiary structures of the ligand-binding domain of different steroid receptors are highly similar. 8-11 Second, detailed sequence, structural, and functional analyses strongly support the hypothesis that the ancestral steroid receptor bound estrogens and specificity for other steroids evolved by serial and parallel duplications of the ancestral gene, mutation of nucleotides coding for specific amino acids, and structural and functional divergence of the paralogs. 12-14 Finally, steroid hormone receptors are nuclear receptors unique to the chordate lineage, indicating that they originated when the first chordates evolved.

The mechanisms of mineralization are not fully understood. Fluorescent, low-molecular weight compounds such as tetracycline or calcein bind strongly to bone mineral, when administered for short periods. They then accumulate in narrow bands in the new bone. [28] These bands run across the contiguous group of bone-forming osteoblasts. They occur at a narrow (sub- micrometer ) mineralization front. Most bone surfaces express no new bone formation, no tetracycline uptake and no mineral formation. This strongly suggests that facilitated or active transport , coordinated across the bone-forming group, is involved in bone formation, and that only cell-mediated mineral formation occurs. That is, dietary calcium does not create mineral by mass action.

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