Chromosome Enumeration Probes are designed for identification and enumeration of human chromosomes in interphase cells and as an adjunct to standard karyotyping in metaphases. These probes will produce sharp, bright signals specific for each individual chromosome. 

As several chromosomes share the same repetitive sequences resulting in cross-hybridization signals, they cannot be differentiated by centromere specific probes. Instead, these chromosomes can be identified by direct labeled Specific Probes hybridizing in close proximity to the respective satellite DNA sequences or to other chromosome specific loci.

The 21q22 Probe hybridizes to the so-called Down Syndrome Critical Region on 21q22.13-q22.2 commonly duplicated in cases with partial trisomy 21. The region at 21q22 includes the DSCR4 (Down syndrome critical region 4). Since chromosomes 13 and 21 share the same repetitive sequences, they cannot be differentiated by probes detecting centromere specific repeats.