The present invention provides modified oligonucleotide primers designed to incorporate a cleavable moiety so that a 3 portion of the primer (linked to an extension product) can be released from an upstream 5 portion of the primer. Upon selective cleavage of the cleavable site, primer extension products that contain about five or fewer base pairs of the primer sequence are released, to provide more useful sizing and sequence information per fragment than extension products containing the entire primer.