Analysis of TdT-mediated N-region insertions at signal and coding joints. (a) pSJ-F1 and pCJ-F1 plasmid substrate. (b) Fifteen illustrative N-region insertion sequences are shown for signal or coding junctions. Nucleotide insertions are shown between the adjacent sequences in the recombined joint. G + C = 75% and A + T = 25% for total number of N-region nucleotides analyzed at signal joints. G + C = 67% and A + T = 33% for total number of N-region nucleotides analyzed at coding joints. A 2 × 2 contingency table analysis of G–C composition of 40 independent recombinants containing N-region insertions is shown. For the purposes of G–C analysis, the N region was divided into left and right halves, with the central nucleotide omitted for odd numbers of nucleotides. The P value determined for equivalent N-region addition to each strand versus no effect, according to the Fisher's exact test, was P < 0.0001 for signal joints and P = 0.105 for coding joints.