Using a biochemical approach (evaluation of esterase D activity) and recombinant DNA techniques (in situ and filter hybridization with specific DNA probes) a glioblastoma cell line with karyotypical nullisomy 13 was shown to contain several chromosome #13-specific sequences. They were assigned to a marker chromosome. This finding suggests that cytogenetic descriptions of deletions or chromosome losses will gain considerable power of the statement when supplemented by molecular analysis, particularly since, by now, DNA probes have been accumulated for all chromosomes and all their subfragments.