In 1857 Moore, and in Japan in 1929 Esaki reported for the first time, since then, about 250 cases have been reported by 1998.

Frequency of development:
in anorectal malignant tumor 0.38%
in all malignant melanoma 4.6%
Common age of onset:60s
Male:Female 1:2
Common site:about 90% is in the anorectal junction
Earliest symptom:The majority is anal bleeding. Or, anal pain, anal tumor and so on.
Gross form:About 80% is tumor type or polyp-like elevated type. About 20% is ulcer type.
Color tone:However many of them display black, about 20% is amelanotic or hypochromic.