Through linkage analysis and the identification of structural chromosome rearrangements, a number of disease genes have been mapped to the pericentromeric region of the long arm of chromosome 13. Structural rearrangements, or deletions, of the 13q12 region have been implicated in a range of myeloproliferative neoplasms, and other haematopoietic malignancies. In particular, seven cases of a t(8;13) (p11; q12·1) rearrangement have been noted in patients with an atypical myeloproliferative disorder associated with T-cell leukemia and eosinophilia. We have previously identified a CEPH megaYAC, 943E4, which crosses the translocation breakpoint in archival tumour samples from two patients with this t(8;13) translocation. As an initial step in the characterisation of this translocation breakpoint, we have generated a fine structure physical map of this 1·9 Mb YAC. We have used the method of YAC fragmentation to generate a series of deletion constructs of known size, which provide discreet physical landmarks convenient for mapping genetic markers along the 943E4 YAC. Analysis of these deletion constructs defined the order of ESTs and microsatellite markers in 943E4 as: cen-NIB1257-(ATP1AL1/D13S283)- D13S179E-(D13S504E/D13S505E)-D13S824E-D13S182E-D13S221-tel. These markers have also been assigned to physically defined regions relative to the fragmented YAC endpoints and a derived NotI restriction map.