BMC Cancer 2006, 6:2 http://www.biomedcentral.com/1471-2407/6/2 FCiogpuyrenu5mber of BACs in cells from recurrent tumors resistant to 10 µ g/ml BCNU Copy number of BACs in cells from recurrent tumors resistant to 10 µ g/ml BCNU. Tumor DIR did not have more than 4 signals for any BAC probe. cytogenetic analyses echoed the heterogeneity and com- plex genetics of these tumors identified by standard cytogenetics. The majority of malignant gliomas recur at or near the site of the original tumor, and it is thought that recurrent tumor arises from cells that survived the patient's treatment. Thus, cells in the recurrent tumor can be thought of as being selected for resistance in vivo. This is further suggested by cells in the recurrent tumor that carry markers identified in the primary untreated cells, i.e. del(17)(p11.2) found in the LX and LXR cells shown in Figure 2. We have demonstrated that the heterogeneity present in primary tumors is markedly reduced in recur- rent tumors suggesting that among the large number of genetic subpopulations, several may contain resistance and/or survival factors that permit selection and propaga- tion. However, despite the presence of complex karyo- types with multiple abnormalities, cells selected for BCNU resistance in vitro or in vivo have a specific genetic characteristic – near diploid with over-representation of chromosome 7 and part, or all of chromosome 22 [2,10]. While in recurrent tumors whole copies of chromosome 22 may be under-represented, FISH analysis demon- strated that numerous fragments of chromosome 22 were inserted in derivative and marker chromosomes. This was not found for other chromosomes when analyzed in a similar manner, and additional fragments of all other Page 8 of 10 (page number not for citation purposes)