# Chemistry and Imaging on iSeq 100
The iSeq 100 System combines complementary metal-oxide semiconductor (CMOS) technology with innovative one-channel sequencing by synthesis (SBS) chemistry to deliver high-accuracy data in a compact system.
The system uses a patterned flow cell with nanowells fabricated over a CMOS chip (Figure 1).
Figure 1: Illumina CMOS Flow Cell.
Clustering and sequencing occur in the nanowells with direct alignment of single clusters over each photodiode (pixel). Using a CMOS sensor embedded in the consumable is a simple and fast detection method.
Unlike four-channel SBS chemistry, where sequencers use a different dye for each nucleotide, the iSeq 100 System uses one dye, two chemistry steps, and two imaging steps per sequencing cycle (Figure 2). In one-channel chemistry, adenine has a removable label and is labeled in the first image only. Cytosine has a linker group that can bind a label and is labeled in the second image only. Thymine has a permanent fluorescent label and is therefore labeled in both images, and guanine is permanently dark.
Figure 2: One-Channel Chemistry by schematic representation.
Nucleotides are identified by analysis of the different emission patterns for each base across the combination of Image 1 and Image 2 that are processed by image analysis software to identify which bases are incorporated at each cluster position.
Figure 3: Image analysis during SBS Chemistry with One-Channel Detection.
The iSeq 100 System uses during cluster generation, a proprietary ExAmp chemistry to ensure that each well in the flow cell generates a single, clonal cluster. During each imaging step, light emissions are detected by the CMOS photodiodes.
For more information, see the Sequencing by Synthesis page [here](https://www.illumina.com/science/technology/next-generation-sequencing/sequencing-technology/2-channel-sbs.html).
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