Abstract: Oligonucleotides of formula(X).sub.a -(6N-palindrome).sub.n -(Y).sub.b ( 1)wherein:"6N-palindrome" represents TACGTA, ACGTAT, CGTATA, GTATAC, TATACG or ATACGT (read in the conventional notation with the 5'-end at the left);n is an appropriate integer of at least 2;X is a length of appropriate 5'-end or tailing DNA and a is 0 or 1; andY is a length of appropriate 3'-end or heading DNA and b is 0 or 1;or any labelled form thereof or any derivative thereof which retains the ability of the 6N-palindrome sequence to anneal or hybridize to its complementary sequence are useful in assay of encephalopathies such as scrapie, BSE or CJD. Preferably they are used as PCR primers to amplify DNA in the sample. PCR product can be detected by restriction with an appropriate enzyme and comparing the restricted and unrestricted products.