Fig. 1

a Clinical presentation of patient 1#: The GCMN lesion was located on the left upper extremity, with clear borders, no hirsutism, and brownish-black color. b Clinical presentation of patient 2#: The GCMN lesion was located above the elbow and left shoulder of the left upper extremity, with a well-defined, dark, hairy lesion. c The upper extremity of patient 1# with GCMN was measured for circumference using a soft ruler. d The healthy upper extremity of patient 1# was measured for circumference using a soft ruler. e The upper extremity of patient 2# with GCMN was measured for circumference using a soft ruler. f The healthy upper extremity of patient 2# was measured for circumference using a soft ruler. GCMN: Giant congenital melanocytic nevus