In Argentina, the collection of DNA in criminal investigation procedures occurs at two stages. First, DNA may be collected at the enquiry stage, during which, under the direction of a judge, evidence is assembled in relation to the charge and the defence. Secondly, DNA may be collected during the criminal prosecution procedure. At this stage there is an opportunity for the parties to challenge, on an equal footing, the DNA evidence already obtained or introduced. The degree of force which can be used to obtain DNA samples (from hair, blood, saliva or other human tissues) is determined by the principle of proportionality. Some medical examinations could extend to DNA analysis which is primarily for the purpose of detecting illness amongst prisoners, military personnel or employees but including those who are going to be married, where there are epidemics or skin grafts. Laboratories and experts who carry out DNA tests must be licensed.