Alus and B1s are short interspersed repeat elements (SINEs) derived from the 7SL 
RNA gene. Alus and B1s exist in the cytoplasm as non-coding RNA indicating that 
they are actively transcribed, but their function, if any, is unknown. 
Transcription of individual SINEs is a prerequisite for retroposition, but it is 
also possible that individual Alu and B1 elements have some cellular functions. 
Previous studies suggest that transcription of Alu elements depends on the 
presence of an RNA polymerase-III bipartite promoter and the poly-A tail. 
Sequencing of small RNAs has demonstrated that the members of the Y and S 
subfamily are expressed. We analyzed almost one million Alu sequences longer 
than 200 nucleotides for the presence of RNA polymerase-III bipartite promoter 
sequences. More than half contained a promoter indicating some potential for 
expression. We searched 7.7 million human EST sequences in dbEST for the 
presence of Alu non-coding RNAs and found evidence for the expression of 452. 
Analysis of mouse spermatogenic dbEST libraries revealed an apparent 
relationship between the level of differentiation and the level of B1-related 
sequences in the EST library.