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There are four forms, which vary based upon the arrangements of the "P" and "A" groups (in clockwise order):
- In the "I" variety (i.e. uroporphyrinogen I), the order repeats four times: AP-AP-AP-AP.
- In the "III" variety (i.e. uroporphyrinogen III), the fourth is reversed: AP-AP-AP-PA.
- The "II" and "IV" varieties can be created synthetically, but do not appear in nature.
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