MC1R and freckles: In contrast, αMSH has not been identified as an intrinsic melanogenic cytokine in UVB-melanosis based on the following evidence: on the tail skin of C57BL/6J mice-aa/ee (recessive yellow), which have a mutation in the melanocortin 1 receptor (MC1R) and do not respond to MSH, UVB irradiation induced a distinct hyperpigmentation concomitant with an increased number of epidermal melanocytes and an increased activity of tyrosinase [21].