They also demonstrated that L1 RNA transcribed in egg or sperm cells can be carried over through fertilization and integrate during embryogenesis, an interesting example of heritability of RNA independent of its encoding DNA, creating somatic mosaicism during mammalian development.
Soma are all cells other than egg or sperm cells.
A cell mosaic is an insertion that occurs after fertilization in which some cells have the insertion and others don't within the same tissue type.
The mosaicism suggests a role for L1 in carcinogenesis and other diseases; for example cancerous growth may be initiated if insertions happen near an oncogene.