Approximately 30% of human patients with osteopetrosis have unrecognized molecular defects [15,38]; thus, it may be worthwhile to examine these orphan human osteopetrotic syndromes for mutations in SLC4A2. However, it has also been suggested that such human SLC4A2 mutations may result in embryonic lethality in humans due to ubiquitous expression of this protein, and as a result, will likely remain undetected [25]. This evidence concerns the gene SLC4A2 and osteopetrosis.