CHEK2 and breast cancer: Although higher frequencies of actionable gene variants are reported in those at particularly young ages (<40 years) particularly for TP53, the PV rates of ATM and CHEK2 do not appear to be strongly related if at all to age-at-onset, although a small effect was seen for CHEK2 in two studies.1 2 Very few studies have concentrated testing on women with very early onset breast cancer.