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From 2012.igem.org

Currently, the traditional method of synthetic biology uses proteins as on/off signals for cell based computing. The potential of these circuits is fundamentally limited by the number of proteins that can be orthogonally and simultaneously expressed.

Imagine a circuit that senses for potential cancer cells and then produces a fluorescent protein that allows the cells to be easily identified by a doctor or surgeon. What would this require? The first step, a cancer cell sensor, can be achieved by creating an mRNA sensor. The state of the art circuit with this function, (1 Multi-Input RNAi-Based Logic Circuit for Identification of Specific Cancer Cells. Xie et al. Science 2011) (PIC) requires at least five composite parts for sensing high and low mRNA concentrations.