JAK1 and JAK3 immunoreactivity scores were significantly higher in the tumor than in the patch and plaque stages of MF (P7 = 0.001 and 0.004, respectively).<h4>Study limitations</h4>This study is limited by the relatively small sample size of some clinical variants of mycosis fungoides, particularly advanced forms such as erythrodermic and tumor-stage MF. Here, JAK3 is linked to mycosis fungoides.