The Alu sequence family comprises the major dispersed repeat sequences of rodent 
and primate genomes, numbering greater than 300,000 copies in the human haploid 
genome. The function of these elements is unknown. The sequences can be 
transcribed by RNA polymerase III and represent a substantial fraction of total 
heterogeneous nuclear RNA. Alu sequences can be found both in the flanking 
regions and within the transcription units of several well-characterized genes. 
Here we show that some members of the mouse B1 Alu sequence family encode a 
small cytoplasmic RNA. The mouse B1 sequence is congruent to 130 nucleotides 
long and shows homology with the monomeric units of the dimeric 300-nucleotide 
primate sequence. By means of microinjection studies in the Xenopus laevis 
oocyte, we have elucidated a novel pathway leading to the appearance of a 
processed B1-type Alu RNA species in the cytoplasm. The abundance of this small 
Alu RNA differs between various mouse tissues, suggesting a role in 
tissue-specific gene expression.