CDKN2A and melanoma: SN stains strongly and diffusely for p16, but sometimes a mosaic pattern is also interpreted as a sign of benignity; on the contrary, complete absence or absence in sizable areas of the tumor, due to homozygous loss of 9p21, is an indication of loss of p16 expression and is a useful ancillary study to suspect melanoma (68, 81, 82).