A series of dATP and dCTP nucleotide analogs have been synthesized which are modified by attachment of aliphatic linkers containing a functional group to the amino-nitrogen at the hydrogen bonding positions of the bases, that is, at the 6-position of adenine and the 4-position of cytosine. These nucleotides are incorporated into DNA probes by standard nick-translation protocols. DNA probes labeled with biotin derivatives of these nucleotides are effectively hybridized to target DNA sequences and can be detected by a streptavidin and calf intestinal alkaline phosphatase conjugate with a sensitivity (0.25 pg DNA) sufficient for reproducible and rapid detection of single copy genes in a Southern blot of mammalian DNA.Also, a procedure has been developed to allow reprobing of nylon filters that have been hybridized with biotinylated probes and developed with the streptavidin/alkaline phosphatase conjugate and a standard dye system.