Direction : write the letter of the underlined part that is grammatically incorrect.
A gene transfer (1) vector using Tol2 has already been (2) established in fish. Foreign DNA fragements inserted in Tol2 can be efficient (3) delivered to the chromosomes by transposition. The latest (4) version of the vector contains, between the Tol2 terminal regions, a bacterial drug-resistance gene and a plasmid replication origin.
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