We hypothesized that MMP‐9 (gelatinase B) was a strong candidate in CD46 ectoderm shedding; first, because microarray analysis of CD3+CD46‐activated CD4+ T cells revealed elevated levels of MMP9 transcripts 11, and second, because anti‐MMP‐9 autoantibodies have been reported in SLE 27. Here, MMP9 is linked to systemic lupus erythematosus.