Complex sentence: Many of these cancers express the normal HMGA2 protein, but the mature mRNA transcript is truncated, missing a portion of the 3' UTR that contains the critical let-7 complementary regions.
Split into simpler sentences: Many of these cancers express the normal HMGA2 protein, but the mature mRNA transcript is truncated (shortened). To be specific, the transcript is missing a portion of the 3' UTR that contains the critical let-7 complementary regions.