Sir

It was disappointing that your reporter contributed further confusion to the issue of cow brains being tested instead of sheep brains for signs of BSE (“Brain mix-up leaves BSE research in turmoil”, Nature 413, 760; 2001) by failing to contact the Laboratory of the Government Chemist (LGC) before declaring that our findings in this case were open to dispute.

To set the record straight, LGC is an independent, non-Government laboratory committed to the highest standards of analytical quality.

We have developed, validated and deployed DNA-based tests for the speciation of meat for many years, often in cases of commercial misdescription of processed meat. The Department for the Environment, Food and Rural Affairs (DEFRA) commissioned LGC to check the species identity of the brain samples that were used in the BSE experiments conducted by the Institute of Animal Health (IAH).

Our tests used two different extraction methods, three methods for qualitative identification and a full battery of controls, blanks and authentic samples.

We were aware that the IAH samples had already been subjected to a number of cycles of freeze-thawing, so the experiments were specifically designed to be able to detect degraded DNA (about 220 base pairs). The identification of beef DNA was unambiguous. (Our report to DEFRA is available on the LGC website, http://www.lgc.co.uk.)

The assertion in Nature that the storage conditions of the samples in transit to LGC could have affected the results does not bear scientific examination. DNA is a robust molecule, and it would be a remarkable sequence of freezing and thawing that turned sheep DNA into beef.