A method for determining whether a mammalian subject having a malignant melanoma belongs to a first or a second group, wherein the prognosis of subjects of the first group is better than the prognosis of subjects of the second group is provided. The method comprises the steps of: evaluating an amount of RBM3 protein in at least part of a sample earlier obtained from the subject and determining a sample value corresponding to the evaluated amount; comparing said sample value with a predetermined reference value; and if said sample value is higher than said reference value, concluding that the subject belongs to the first group; and if said sample value is lower than or equal to said reference value, concluding that the subject belongs to the second group.