In conclusion, the present study provides additional evidence for the existence of an alternative FeLV infection course that leads to development of antibodies without involvement of the bone marrow. The results may be important for the surveillance of the FeLV status of a cattery or a cat population. Most likely a cattery cannot be considered free of FeLV as long as cats with FeLV-reactive antibodies are present. The potential for overt infection cannot be neglected. A large and accurate collection of peptidase cleavages in the MEROPS database Peptidases are enzymes that hydrolyse peptide bonds in proteins and peptides. Peptidases are important in pathological conditions such as Alzheimer's disease, tumour and parasite invasion, and for processing viral polyproteins. The MEROPS database is an Internet resource containing information on peptidases, their substrates and inhibitors. The database now includes details of cleavage positions in substrates, both physiological and non-physiological, natural and synthetic. There are 39 118 cleavages in the collection; including 34 606 from a total of 10 513 different proteins and 2677 cleavages in synthetic substrates. The number of cleavages designated as ‘physiological’ is 13 307. The data are derived from 6095 publications. At least one substrate cleavage is known for 45% of the 2415 different peptidases recognized in the MEROPS database. The website now has three new displays: two showing peptidase specificity as a logo and a frequency matrix, the third showing a dynamically generated alignment between each protein substrate and its most closely related homologues. Many of the proteins described in the literature as peptidase substrates have been studied only in vitro. On the assumption that a physiologically relevant cleavage site would be conserved between species, the conservation of every site in terms of peptidase preference has been examined and a number have been identified that are not conserved. There are a number of cogent reasons why a site might not be conserved. Each poorly conserved site has been examined and a reason postulated. Some sites are identified that are very poorly conserved where cleavage is more likely to be fortuitous than of physiological relevance. This data-set is freely available via the Internet and is a useful training set for algorithms to predict substrates for peptidases and cleavage positions within those substrates. The data may also be useful for the design of inhibitors and for engineering novel specificities into peptidases. Database URL: http://merops.sanger.ac.uk Peptidases (proteases or proteinases) are enzymes that hydrolyse the peptide bonds between amino acids in a protein or peptide chain. Hydrolysis of such bonds is required for removal of targeting signals (signal and transit peptides (1), ubiquitin (2), SUMO (3) and NEDD8 (4) peptides), the release of a mature protein from its precursor (5) , the switching off of a biological signal by degradation of the signal protein (6) , and for widespread catabolism of proteins for recycling of the amino acids. When proteolysis occurs unchecked, then diseases can result, such as Alzheimer's (7) , osteoarthritis (8) , emphysema (9) , tumour invasion (10) and acute pancreatitis (11) . Pathogens use peptidases to enter the host and to degrade host proteins for food (12) .