The MSD1 region of neural cell adhesion molecule (NCAM) was originally described as being spliced into the 120-kDa isoform of NCAM isolated from muscle. The 105 bp region is inserted between exons 12 and 13 and actually consists of three separate exons, MSD1a, MSD1b and MSD1c of 15, 48, 42 bp, respectively. In addition, a further exons consisting of a single triplet has been designated MSD1d, making the full insert size 108 bp. As the MSD1 region was originally described as being selectively expressed in muscle tissue, we have investigated whether it is also present on tumours of rhabdoid origins and whether its presence can be used as the diagnostic marker to distinguish other small round cell tumours of childhood, such as neuroblastoma. Using a variety of human tumour cell lines, we demonstrated the presence of the MSD1 region on all rhabdomyosarcomas investigated. However, neuroblastoma cell lines only expressed subcompartments of the MSD1 region. The MSD1c exon was not spliced into the NCAM molecules isolated from any of the neuroblastoma cell lines investigated. On the basis of this finding, it appears that neuroblastoma and rhabdomyosarcoma can be distinguished by the expression of MSD1c mini-exon. Further studies are underway to attempt to define a monoclonal antibody that recognises the region, using mice immunised with synthetic peptides, and to confirm the finding using fresh biopsy material.