Though the 9 Å model provides a partial explanation for why mRNA pseudoknots promote programmed À1 ribosomal frameshifting (À1 PRF) more efficiently than simple *To whom correspondence should be addressed. Tel: +1 301 405 0981; Fax: +1 301 314 9489; Email: dinman@umd.edu ª The Author 2005. Published by Oxford University Press. All rights reserved.