Of the 60 SDHD tumours investigated, four tumours showing retention of chromosome 11 showed SLC22A18 and CDKN1C expression levels comparable to levels in tumours showing loss of chromosome 11, suggesting loss of protein expression despite chromosomal retention.Our data strongly suggest that SLC22A18 and/or CDKN1C are tumour modifier genes involved in the tumourigenesis of SDHD-linked paraganglioma.