Described herein is a new approach in which a nucleic acid species of interest (e.g. a chromosome) containing multiple unique target sequences is detected using multiple specific probes that are amplified by rolling circle amplification and detected. Multiple probes are used to provide a detectable signal, where the magnitude of the signal is proportional to the number of probes recognising their target sequences. Individual signals from the plurality of probes are converted into a single cumulative detectable signal, amplifying the individual signals through the multiplex probing. Ten or more probes produce a signal amplification of ten-fold or more. The generated signals depend on correctly reacted probes upon target recognition, using sequence specific hybridisation and enzymatic catalysis to generate specific products from which the signal is obtained.