Like all 10 minor receptor group human rhinoviruses (HRVs), HRV23 and HRV25, previously classified as major group viruses, are neutralized by maltose binding protein (MBP)-V33333 (a soluble recombinant concatemer of five copies of repeat 3 of the very-low-density lipoprotein receptor fused to MBP), bind to low-density lipoprotein receptor in virus overlay blots, and replicate in intercellular adhesion molecule 1 (ICAM-1)-negative COS-7 cells. From phylogenetic analysis of capsid protein VP1-coding sequences, they are also known to cluster together with other minor group strains. Therefore, they belong to the minor group; there are now 12 minor group and 87 major group HRV serotypes. Sequence comparison of the VP1 capsid proteins of all HRVs revealed that the lysine in the HI loop, strictly conserved in the 12 minor group HRVs, is also present in 9 major group serotypes that are neutralized by soluble ICAM-1. Despite the presence of this lysine, they are not neutralized by MBP-V33333 and fail to replicate in COS-7 cells and in HeLa cells in the presence of an ICAM-1-blocking antibody. These nine serotypes are therefore "true" major group viruses.Human rhinoviruses (HRVs), the main causative agents of common cold, were originally classified as acid-sensitive picornaviruses (34). Later, based on competition for cellular binding sites, two different groups of viruses using nonidentical receptors for cell attachment were defined within the genus Rhinovirus (18). Subsequently, 24 (1) and finally 100 serotypes (counting the subtypes HRV1A and HRV1B as one strain) were assigned to the two receptor groups by using cross-competition and inhibition of cell binding by a monoclonal antibody recognizing intercellular adhesion molecule 1 (ICAM-1), the receptor of the major group of HRVs (32,33,35). According to these reports, 90 serotypes bind ICAM-1, whereas both subtypes of HRV1 (HRV1A and HRV1B) and 8 other serotypes were categorized as belonging to the minor group; they were later shown to use members of the low-density-lipoprotein receptor (LDLR) family for cell entry (12,20,36). HRV87 was noted to be an exception in that it binds a sialylated membrane protein. Based on comparison of nucleotide sequences in several genomic regions, HRV87 was subsequently classified as an acid-sensitive enterovirus and found to be a prime strain of enterovirus 68 that uses decay-accelerating factor as a receptor (2,27). By phylogenetic analysis of capsid protein VP4/VP2 coding sequences of all HRV prototype strains, 74 HRV serotypes, including all the minor receptor group ones, were classified as HRV species A, while the 25 remaining serotypes form HRV species B (27). In a paper from the Colonno group, primary data on the competition with an ICAM-1-blocking antibody were not explicitly presented for all serotypes, in particular not for HRV23 and HRV25, and the presumed allocation of all HRVs to either group was depicted only in the form of a summarizing figure (35). Despite this, it was generally accepted that 90 serotypes, including HRV23...