L1 activity has been observed in numerous types of cancers, with particularly extensive insertions found in colorectal and lung cancers. It is currently unclear if these insertions are causes or secondary effects of cancer progression. However, at least two cases have found somatic L1 insertions causative of cancer by disrupting the coding sequences of genes APC and PTEN in colon and endometrial cancer, respectively.
Quantification of L1 copy number by qPCR or L1 methylation levels with bisulfite sequencing are used as diagnostic biomarkers in some types of cancers. L1 hypomethylation of colon tumor samples is correlated with cancer stage progression. Furthermore, less invasive blood assays for L1 copy number or methylation levels are indicative of breast or bladder cancer progression and may serve as methods for early detection.