An oligonucleotide, primer or probe comprises the nucleotide sequences of any of SEQ ID NO. 5, 6, 7, 2, 3, 4, 8, 9, 11, 12, 13, 14, 15, 16, 17, 18, 19 or 25. The oligonucleotides are useful for the detection of the methylation status of a gene, in particular the MAGE-A3 gene. The oligonucleotides are useful in primer pairs, kits and methods for determining the methylation status of the MAGE-A3 gene and for diagnosing cancer, directing therapy and selecting subjects for treatment. The primer or probe can comprise a loop or hairpin structure and can be used in real-time methylation specific PCR.