A method for quantitatively detecting in a sample from an individual an amount of a species of DNA originating from cells of a given type, which sample comprises said species of DNA in admixture with differently methylated DNA not originating from cells of said type wherein said species of DNA originates from cells of a foetus and/or the placenta of a foetus and said sample is from a pregnant female; said method comprising the steps: (a) treating the DNA present in said sample with a reagent that differentially modifies methylated and non-methylated DNA, wherein said reagent comprises at least one methylation sensitive restriction enzyme; (b) quantitatively detecting in said sample the presence of methylation in said species of DNA at one or more differentially methylated region(s) (DMR(s)) that is(are) differently methylated between said species of DNA and the DNA not originating from cells of said type, the modification of DNA of such DMR by said reagent is sensitive to methylation of DNA, wherein the presence of methylated DNA at said DMR indicates the presence of said amount of species of DNA in said sample and the absence of methylated DNA at said DMR indicates the absence of said species of DNA in said sample, wherein said DMRs is(are) hyper-methylated in foetal DNA and hypo-methylated in maternal DNA, and wherein said DMR(s) comprise(s) one, two, three or more site(s) of methylation recognised by said methylation sensitive restriction enzyme; and (c) quantitatively detecting an amount of total DNA present in said sample using at least one other region that does not comprise a site of methylation recognised by said methylation sensitive restriction enzyme, wherein, said other region is located between about 20bp and about 20kb upstream or downstream of said DMR; and wherein said detection in step (b) and said detection in step (c) are made using the same aliquot of DNA of said sample, and in the same reaction/detection vessel, and effectively simultaneously for such DMR(s) and other region(s), and using: (x) a detectable label(s) for said DMR(s); and (y) a different detectable label(s) for said other region(s); and wherein: as part of said detection in step (b) and step (c), each DNA region comprising said DMR(s) and said other region(s) are amplified; and each detectable label used in step (b) and step (c), is a fluorescent label; and said detection in step (b) comprises real-time quantitative probe-based PCR using at least one labelled probe specific for one of said DMR(s); and said detection in step (c) comprises real-time quantitative probe-based PCR using at least one labelled probe specific for one of said other region(s). (a) treating the DNA present in said sample with a reagent that differentially modifies methylated and non-methylated DNA, wherein said reagent comprises at least one methylation sensitive restriction enzyme; (b) quantitatively detecting in said sample the presence of methylation in said species of DNA at one or more differentially methylated region(s) (DMR(s)) that is(are) differently methylated between said species of DNA and the DNA not originating from cells of said type, the modification of DNA of such DMR by said reagent is sensitive to methylation of DNA, wherein the presence of methylated DNA at said DMR indicates the presence of said amount of species of DNA in said sample and the absence of methylated DNA at said DMR indicates the absence of said species of DNA in said sample, wherein said DMRs is(are) hyper-methylated in foetal DNA and hypo-methylated in maternal DNA, and wherein said DMR(s) comprise(s) one, two, three or more site(s) of methylation recognised by said methylation sensitive restriction enzyme; and (c) quantitatively detecting an amount of total DNA present in said sample using at least one other region that does not comprise a site of methylation recognised by said methylation sensitive restriction enzyme, as part of said detection in step (b) and step (c), each DNA region comprising said DMR(s) and said other region(s) are amplified; and each detectable label used in step (b) and step (c), is a fluorescent label; and said detection in step (b) comprises real-time quantitative probe-based PCR using at least one labelled probe specific for one of said DMR(s); and said detection in step (c) comprises real-time quantitative probe-based PCR using at least one labelled probe specific for one of said other region(s).