The int-2 gene, a member of the fibroblast growth factor (FGF) super-gene family, has previously been shown to be amplified in 16% of the 110 human breast tumors examined. In order to characterize the amplification unit containing the int-2 gene (chromosome 11q13), the same panel of breast tumors was screened for possible amplifications of other markers mapping between 11q11 and 11q24. Out of the eight additional genes analysed, simultaneous amplification of bcl-1 (11q13, a locus involved in hematopoietic malignancies) and hst (11q13, another member of the FGF family) was observed in 17/18 tumors with increased copy number of the int-2 gene. A single breast tumor showed amplification of int-2 oncogene only. Neither the bcl-1 nor the hst locus was individually amplified in any of the tumor DNAs examined.