Fig 1. Only some of the maternally methylated imprints look like retrotransposons, but all are controlled by KRAB-ZF proteins.

(A) Table showing progress in clarifying the transposon-imprinting link. Initial studies looked at individual genes (in italics) from the approximately 100 known; later work concentrated on ICRs (in bold) that regulate multiple genes: there are approximately 19 ICRs. Although a relatively small number of imprinted genes show clear evidence of retrotransposition, almost all are bound by, and many dependent on, zygotically expressed KRAB-ZFP57 for methylation maintenance. Recently, the Trono and Ferguson-Smith labs have demonstrated a clear role for a second KRAB-ZF protein (called ZNF445) in regulating a partially overlapping set of ICRs. If maternally expressed stores of ZFP57 are also depleted, even more ICRs are affected, thus covering 18/19 ICR. The one remaining ICR is Peg10, which is clearly related itself to the sushi retrotransposon class (row 2) and may be regulated by an as-yet-undiscovered KRAB-ZF protein. (B) Schematic summarizing the data shown in (A): the 3 smallest circles refer to genes, the larger to ICR, as above. Together, these studies clearly link all imprinted loci with host defense. CG, cytosine-guanine; ICR, imprint control region; KRAB-ZF, Krüppel-associated box–zinc finger; ZFP57, zinc finger protein 57; ZFN445, zinc finger 445, ZNF445.