Digital RMD for noninvasive detection of X-linked diseases in maternal plasma. (A) Digital RMD for X-linked diseases. The male fetus inherits either the mutant allele (M) or the wild-type allele (N) from his mother, leading to an overrepresentation of the M or the N allele, respectively, in maternal plasma. (B) SPRT for hemophilia detection. Samples with mutant allele proportion (P_r) above the upper boundary and below the lower boundary are classified as mutant and wild-type, respectively. Samples with P_r between 2 curves are unclassifiable and require additional digital PCR analysis.