The present invention generally relates to microfluidics and, in some embodiments, to the determination of cells. In some aspects, primers able to introduce restriction sites into certain amplified nucleic acids are used. For example, the primers may introduce restriction sites into normal (wild-type) nucleic acids, but be unable to introduce restriction sites into mutant nucleic acids, e.g., due to a mismatch in the nucleic acid sequences caused by the mutant. After amplification, the nucleic acids may be exposed to a suitable restriction enzyme, which may cleave normal nucleic acids but not the mutant nucleic acids. In this way, mutant nucleic acids may be relatively quickly identified. In some embodiments, cells may be contained within microfluidic droplets and assayed to determine the mutant cells. In certain cases, for example, the nucleic acids may be amplified within droplets and attached to suitable tags, e.g., prior to breaking or merging the droplets and sequencing of the nucleic acids.
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