In situ hybridization of a human chromosome 3 paint to a metaphase spread of a flying fox (Pteropus poliocephalus). The human chromosome paint was made by flow-sorting and subsequent degenerate-oligonucleotide-primed (DOP) PCR. The probe was labeled with biotin and detected with avidin-fluorescein isothiocyanate (FITC). Note that only two signals are detected, revealing that human chromosome 3 is entirely syntenic with a single flying fox chromosome. The unhybridized section of this chromosome in the flying fox is homologous to human 21. The association of human chromosomes 3+21 is ancestral for placental mammals and is the largest commonly conserved region of synteny in the mammalian genome.