In addition to formal taxonomic systems, viruses are often informally categorized based on shared biological and physical characteristics. Common grouping criteria include host and tissue specificity, capsid shape, presence of unique genes or enzymes, and genome traits such as being naked or enveloped, single-stranded (ss) or double-stranded (ds) DNA or RNA, segmented or nonsegmented, and positive-strand (+) or negative-strand (-) RNA.