is labeled the compositions of the invention comprise a third polynucleotide sequence the third polynucleotide sequence is either unlabeled or labeled with a second detectable marker and is substantially complementary to or substantially identical to the second polynucleotide sequence the presence of the third polynucleotide sequence in the compositions of the invention serves to block the hybridization of the second polynucleotide sequence to any polynucleotide sequences not of interest in the sample being examined which nevertheless are substantially complementary to said second polynucleotide sequence it is believed that this blocking action is achieved in either or both of two ways first the third polynucleotide sequence being substantially complementary to said second polynucleotide sequence can hybridize with the second polynucleotide sequence if said second and third polynucleotide sequences are rendered single stranded and allowed to contact under conditions that permit hybridization second the third polynucleotide sequence being substantially identical to the second polynucleotide sequence can hybridize to any polynucleotide sequences not of interest but complementary to the second polynucleotide sequence and present in the sample being examined it is believed that either of these blocking actions inhibit the liklihood of the generation of a false positive result the second polynucleotide sequence as a sequence contiguous to but distinct from the first polynucleotide sequence in the chromosome in another embodiment of the invention the first polynucleotide sequence which is substantially complementary to and capable of hybridizing to the polynucleotide sequence of interest is covalently.