Sir

Your News in Brief story 'See-through frog offers inside information' (Nature 449, 521; 2007) describes the production of a transparent frog by genetic manipulation. But translucent amphibians were first artificially created 90 years ago.

C. P. McCord and F. P. Allen (J. Exp. Zool. 23, 207–224; 1917) reported that a crude acetone extract of bovine pineal glands fed to Rana pipiens tadpoles caused such a pronounced lightening of their skins that the larger viscera were visible through the dorsal body wall. The tadpoles' transparency varied according to the concentration of pineal extract in the water in which they were swimming.

Some 40 years later, the dermatologist A. B. Lerner followed up these observations and discovered that a pineal indole caused the melanin granules in frog melanocytes to aggregate and the skin to lighten — he named this factor melatonin (A. B. Lerner et al. J. Am. Chem. Soc. 80, 2587; 1958).

So the suggestion made by S. Castroviejo-Fisher and colleagues in Correspondence, of using naturally transparent arboreal glass frogs of the family Centrolenidae for biomedical research, is not without precedent ('Transparent frogs show potential of natural world' Nature 449, 972; 2007).