Misshapen moles aren’t the only sign of deadly melanoma skin cancer.

That’s the latest word from New York University School of Medicine researchers who have found skin cancer usually arises in normal skin, where there is no dark spot or sign of cancer until the melanoma suddenly shows up.

The study, presented at a meeting of the American Society of Clinical Oncology this month, also found melanomas that arise in non-mole areas of the skin tend to be more aggressive and deadly than those that do arise from moles, Live Science reports.

"We find that the ones without a [mole] appear to be more aggressive," said lead researcher David Polsky, M.D., a professor of dermatology, pathology and dermatologic oncology. "We think that there are biological differences" between melanomas that develop within moles and those that develop elsewhere on the skin, and it's possible that these differences underlie the difference in aggressiveness.

The findings are based on an analysis of the medical records of 2,250 people with melanoma.