a. Lifecycle of an intact, autonomous retrotransposon: Retrotransposon DNA is transcribed into RNA (1), which encodes proteins that are needed for reverse transcription and subsequent integration into genomic DNA (2). Using retrotransposonencoded proteins, RNA is reverse transcribed into cDNA that can either exist in an episomal state (3) or can be inserted into genomic DNA (4). b. The number of differentially expressed transposable elements in mouse forebrain from six to twenty months old based on RNA-seq. Differentially expressed transposable elements were grouped into families and represented on the graph. c. Pie charts representing the proportion of subfamily members within each retrotransposon (SINE, LINE, LTR) and DNA transposon (DNA) family. 6 months: n=1 male, 7 females; 12 months: n=4 males, 4 females; 20 months: n=6 males and 5 females. An adjusted P value of <0.05 was considered significant.