To make a probe requires a purified DNA sequence which is complementary
to the gene of interest. Since (1) mRNA can be transcribed which
is complementary to the target gene, (2) is plentiful in lysed red blood
cells, and (3) most contain between 100-200 adenine residues at the
3' end called poly(A) tails,
the above technique utilizing oligo(dT) primers
and reverse transcriptase can be used to produce cDNA probes.