The branch site sequence contributes to splicing efficiency. A) Chimeric forms of Tgif2 were analyzed for splicing of the retained intron. A mutation of the branch site was introduced into the human TGIF2 sequence. HhH: the sequence of the branch acceptor site in the human was altered to that of mouse, HhM: as HhH, with mouse sequence 3' to the retained intron. HMh and MHh: the region 3' of the retained intron consisted of human sequences to base 543 and mouse sequence thereafter. The other regions of the coding sequence were human (H) or mouse (M) as indicated. B) Comparison of the sequences of the splice sites surrounding the retained intron and the branch acceptor site. Sequences from mouse (m), human (h), rat (r), dog (d) and cow (c) are shown with the consensus above. Boxed bases differ from the consensus. C) Pairwise sequence comparison of the coding sequence 3' of the retained intron (upper) or the entire coding sequences (lower) between the five species indicated. Percent identity is shown.