**1.4 Truncations from the other end of insert DNA in BACs**

The genomic DNA insert in BACs is flanked by a loxP site at one end and a mutant lox511 site at the other (24, 26). Modern genome libraries of DNA from numerous organisms such as the mouse, rat, human and zebrafish have used the vector pBACe3.6 or its derivatives [reference (26), link for pTARBAC2.1 vector *http://bacpac.chori.org/ptarbac21.htm*], and all share this characteristic (27-31). End-deletions have been made specifically from the lox511 side of genomic insert by a similar approach using a lox511 Tn10 mini-transposon (24).

It is important to note that second round deletions from the opposite end of insert DNA requires selecting for the transposition event in addition to the BAC DNA: because first round deletions are selected by P1 headfull packaging, all starting BACs for second round deletions would be less than ~110 kb, and hence require selecting for the low frequency of transposition [see reference (23, 24) for detailed discussions].

Fig. 4. Legend: Schematic representation of end-deletions generated from both ends of DNA insert in a BAC clone. The upper panel shows truncations from the lox511 end using a lox511 transposon. The largest clone from that library was then truncated from the loxP end with a loxP transposon. Figure is adapted from reference (24).
