Here, we used the pulmonary melanoma foci assay, which has been commonly used for quick analysis of early metastasis (Vaseghi et al., 2023), to further characterize the higher cancer resistance of the Klf1(K74R) mice with respect to the effects of gender/age/genetic background of the mice and the requirement for the homozygosity of the K74R mutation. The gene discussed is KLF1; the disease is cancer.