Labelling of Probes -The detection of homologous sequences after hybridization with the probe is like finding a needle in the haystack. Therefore, for the success of DNA probe assay, it is necessary to develop simple, safe and sensitive techniques for their use.

As probes transmit no signal of their own, they have to be
(i) either labelled with radioactive isotopes or
(ii) methods are used to couple non radioactive signal molecules to the probes without impairing the hybridization ability of these probes. These signal molecules may include fluorescent antibodies and enzymes that produce colour changes in dyes, and chemiluminescent catalysts.