The embryonic and adult mouse U1 snRNA genes map to different chromosomal loci

The linkage relationships of mouse adult (mU1a) and embryonic (mU1b) U1 snRNA genes were determined by analysis of the strain distribution patterns of two polymorphic variant RNAs, mU1a2 and mU1b3, in several recombinant inbred strain systems. The locus for mU1b3 RNA maps to the U1 gene cluster, Rnu1b, located near the center of chromosome 3, whereas the locus for mU1a2 RNA, Rnu1a2, is located in the proximal region of chromosome 12, tightly linked to D12-1. Moreover, the lack of linkage between Rnu1a2 and the locus for mU1a1 genes on chromosome 11 demonstrates that the mouse genome contains at least three clusters of U1 snRNA genes.