The passage discusses two main topics: the contribution of crossbridges in rigor muscle to stiffness and a new classification system for HIV-1 isolates based on co-receptor use. 1. **Contribution of Crossbridges in Rigor Muscle:** - The text suggests that all crossbridges in a rigor muscle contribute to its stiffness. - If many crossbridges are in a 'slack' state, the calculated duty ratio would be lower, potentially explaining discrepancies in experimental results. - Experiments by Higuchi and Goldman support a small working distance, while the experiment by Yanagida et al. remains difficult to explain. 2. **New Classification System for HIV-1:** - The current classification systems for HIV-1 isolates are imprecise and often misleading. - The proposed new system categorizes isolates based on the co-receptors (CCR5 and CXCR4) used for viral entry. - R5 viruses use CCR5 but not CXCR4, X4 viruses use CXCR4 but not CCR5, and R5X4 viruses can use both co-receptors. - This system provides a more accurate and precise description of viral phenotype, reflecting the ability of isolates to use specific co-receptors. - The classification is flexible and can be expanded to include other co-receptors if necessary. - A record of co-receptor use will be maintained by the Los Alamos National Laboratory Sequence Database.The passage discusses two main topics: the contribution of crossbridges in rigor muscle to stiffness and a new classification system for HIV-1 isolates based on co-receptor use. 1. **Contribution of Crossbridges in Rigor Muscle:** - The text suggests that all crossbridges in a rigor muscle contribute to its stiffness. - If many crossbridges are in a 'slack' state, the calculated duty ratio would be lower, potentially explaining discrepancies in experimental results. - Experiments by Higuchi and Goldman support a small working distance, while the experiment by Yanagida et al. remains difficult to explain. 2. **New Classification System for HIV-1:** - The current classification systems for HIV-1 isolates are imprecise and often misleading. - The proposed new system categorizes isolates based on the co-receptors (CCR5 and CXCR4) used for viral entry. - R5 viruses use CCR5 but not CXCR4, X4 viruses use CXCR4 but not CCR5, and R5X4 viruses can use both co-receptors. - This system provides a more accurate and precise description of viral phenotype, reflecting the ability of isolates to use specific co-receptors. - The classification is flexible and can be expanded to include other co-receptors if necessary. - A record of co-receptor use will be maintained by the Los Alamos National Laboratory Sequence Database.