Site-directed mutagenesis and amino-acid sequencing localized the site of frameshifting to the UUA codon of the HIV-1 slippery sequence (65) . The presence of the GCrich upper stem strongly enhances frameshifting efficiencies. This level of mRNA slippage is enhanced further by the bulge and lower stem. We show here that when the slippery sequence is bound at the decoding site, the ribosome directly influences the spacer sequence in the lower stem that very likely enters the mRNA track. In addition, extended toeprints localized in the upper segment of the lower stem support a physical interaction between this region and the ribosome. These results are in agreement with the importance of the lower stem and bulge regions for the -1 frameshift (8, 32, 66) . It is conceivable that this interaction enhances -1 frameshifting at the particularly slippery UUUUUUA sequence (67) . Outdoor environments and human pathogens in air Are pathogens in outdoor air a health issue at present or will they become a problem in the future? A working group called AirPath - Outdoor Environments and Human Pathogens in Air was set up in 2007 at University College London, UK with the aim of opening new discussion and creating a research network to investigate the science and impacts of outdoor pathogens. Our objective in this paper is to review and discuss the following areas: What is the source of human pathogens in outdoor air? What current, developing and future techniques do we need? Can we identify at-risk groups in relation to their activities and environments? How do we prepare for the anticipated challenges of environmental change and new and emerging diseases? And how can we control for and prevent pathogens in outdoor environments? We think that this work can benefit the wider research community and policy makers by providing a concise overview of various research aspects and considerations which may be important to their work. Low moisture and nutrient levels, combined with high levels of ultraviolet (UV) radiation mean that the atmosphere is inhospitable to microbial life [1] . The huge volume of air outdoors compared to air indoors also helps to dilute the concentration of microbes and reduce the level of exposure. Nevertheless, we need to ask: are pathogens in outdoor air a health issue at present or will they become problematic in the future? The multidisciplinary working group, AirPath was organised to review and discuss the problem of pathogens. Four two-day meetings over a period of 18 months from July 2007 have generated contributions from more than 30 participants, over 20 oral presentations, and various round-table discussions, which were recorded on DVD. In order to summarise the wealth of knowledge contributed by the multidisciplinary group, the panel (the authors of the present paper) has arranged the discussion into five themes: 1. What is the source of human pathogens in outdoor air? 2. What current, developing and future techniques do we need? 3.