Fig. 6.

Proposed classification for genomic copies of snRNAs. Dispersed copies are classified into four groups. Gray rectangles represent snRNA gene or pseudogene, gray broken rectangle represents 3′-truncated snRNA pseudogene, and black rectangle represents non-LTR retrotransposons (SINE or LINE). Black arrowheads represent TSD. In Group IV, LINEs are usually 5′-truncated. Few cases of twin priming between an snRNA and an LINE RNA have generated pseudogene chimeras and are included in Group IV. In this figure, and compared with figure 1 in Denison and Weiner (1982), Group I corresponds to Class I, Group II associates Class II and III, Group III is Class IV. Group IV was not described originally.