Crosslinking oligonucleotides

A process for cross-linking with a target sequence in double stranded DNA, the process comprising:

• contacting in the presence of a recombinase enzyme the double stranded DNA having the target sequence with an oligonucleotide of the formula ##STR10## where B--R--P* represents a nucleotide building block of the oligonucleotide which is substituted or unsubstituted with a reporter group and lacks or includes a radioactive label;
• B in said B--R--P* represents a heterocyclic base component of the nucleotide, R in said B--R--P* is selected from 2-deoxyribofuranosyl, 2-O-alkylribofuranosyl, and 2-deoxy-2-fluororibofuranosyl, and P* in said B--R--P* represents a phosphate group including a phosphate monoester, phosphate diester or phospate triester group, or P* represents a monothioate or dithioate analog of said phosphate groups;
• (A--(L).sub.v).sub.w represents an electrophilic alkylating group wherein L is a leaving group, A is a group that covalently links L to the oligonucleotide and A--L is substantially inert in a cross-linking reaction under conditions that permit hybridization with the target sequence of double stranded DNA, and A--L reacts substantially only after hybridization with the target sequence has occurred;
• n is an integer with the values between 26 and approximately 300,
• v is 1 or 2;
• w is between 1-10,
• and the oligonucleotide includes a continuous sequence of at least approximately 26 nucleotide units which sequence is complementary in the Watson Crick sense to the a target sequence in the double stranded DNA, and wherein at least one A--(L).sub.n group is covalently attached to the continuous sequence that is complementary to the target sequence.