Cysteine is coded by a letter
S
C
Y
E

Options

A. S
B. C
C. Y
D. E

Answer

To determine which letter codes for the amino acid cysteine, we can follow these steps:

1. **Understand Amino Acids**: Cysteine is one of the 20 standard amino acids. Each amino acid is represented by a one-letter code.

2. **Identify the One-Letter Code for Cysteine**: Cysteine is specifically known to be coded by the letter 'C'. This is a standard convention in biochemistry.

3. **Review Other Options**:
- The letter 'S' typically represents serine.
- The letter 'Y' represents tyrosine.
- The letter 'E' represents glutamic acid.

4. **Conclusion**: Since cysteine is represented by the letter 'C', the correct answer to the question is 'C'.

Therefore, the answer is:
**C**

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