Amplification assay with a probe competitor

A method of detecting an allelic form of a target, the method comprising:

• preparing a mixture including (i) at least one first allelic form of a target and a second allelic form of the target, (ii) a same probe capable of binding specifically to each of the first and second allelic forms of the target, and (iii) a competitor that binds selectively to the second allelic form relative to the at least one first allelic form, wherein binding of the competitor to the second allelic form blocks binding of the probe to the second allelic form;
• dividing at least a portion of the mixture into partitions, wherein each partition of only a subset of the partitions contains a first allelic form of the target;
• amplifying the at least one first allelic form of the target in the partitions;
• detecting a signal from a label of the probe while the label is contained by the partitions;
• and
• determining a number of partitions that are positive for the at least one first allelic form, or a number of partitions that are negative for the at least one first allelic form, based on the signal.