Background: The exact origin of the cause of the Severe Acute Respiratory Syndrome (SARS) isstill an open question. The genomic sequence relationship of SARS-CoV with 30 different singlestrandedRNA (ssRNA) viruses of various families was studied using two non-standard approaches.Both approaches began with the vectorial profiling of the tetra-nucleotide usage pattern V for eachvirus. In approach one, a distance measure of a vector V, based on correlation coefficient wasdevised to construct a relationship tree by the neighbor-joining algorithm. In approach two, amultivariate factor analysis was performed to derive the embedded tetra-nucleotide usage patterns.These patterns were subsequently used to classify the selected viruses.Results: Both approaches yielded relationship outcomes that are consistent with the known virusclassification. They also indicated that the genome of RNA viruses from the same family conformto a specific pattern of word usage. Based on the correlation of the overall tetra-nucleotide usagepatterns, the Transmissible Gastroenteritis Virus (TGV) and the Feline CoronaVirus (FCoV) areclosest to SARS-CoV. Surprisingly also, the RNA viruses that do not go through a DNA stagedisplayed a remarkable discrimination against the CpG and UpA di-nucleotide (z = -77.31, -52.48respectively) and selection for UpG and CpA (z = 65.79,49.99 respectively). Potential factorsinfluencing these biases are discussed.Conclusion: The study of genomic word usage is a powerful method to classify RNA viruses. Thecongruence of the relationship outcomes with the known classification indicates that there existphylogenetic signals in the tetra-nucleotide usage patterns, that is most prominent in the replicaseopen reading frames.