Evolution of the diagnostic nucleotide positions from Y to Ya5 Alu elements. Alignment of the five Alu Ya5 diagnostic nucleotides as defined by Shen et al. (1991) and the different Ya1, Ya2, Ya3, and Ya4 elements found in the nr database. For easy reference, individual elements containing different combinations of the diagnostic mutations were numbered consecutively in order of abundance (Ya3.1, Ya3.2, etc.). Ya4.4 elements were considered as Ya5 elements in the first Ya5 subfamily analysis in this paper. The total number of elements found for each subgroup is indicated at left in parenthesis. Potential forward (f) or backward (b) gene conversions are indicated atright. The previously reported order of appearance of Ya5 diagnostic mutations (Shaikh and Deininger 1996) is indicated below. Elements with diagnostic mutations that follow the stepwise hierarchical accumulation are circled.