A fragment length selection gel showing the distribution of DNase I-cleaved fragments is used to identify the upper 40% of the total lane intensity to direct the excision of the packing enriched region. The lane digested with the highest concentration of DNase I (64 U) was chosen for its uniform distribution of fragments around the 5-kb median. This section of the gel was excised, and the DNA was recovered for labeling. (M) λ HindIII DNA mass standard.