The number of factor XIIIa-positive cells per unit area was increased in the lesional skin of Riehl’s melanosis (mean ± SD = 45.66 ± 8.85) compared to perilesional skin (mean ± SD = 23.66 ± 8.11; p < 0.001) and healthy controls (mean ± SD = 10.25 ± 5.22; p < 0.001; Figure 3). Here, F13A1 is linked to freckles.