To solve this problem, we need to understand what Expressed Sequence Tags (ESTs) refer to.$\newline \bullet$ESTs are short sub-sequences of cDNA (complementary DNA) sequences.$\newline \bullet$They are generated by sequencing either one or both ends of an expressed gene.$\newline \bullet$The primary purpose of ESTs is to identify gene transcripts, which are expressed as RNA.$\newline \bullet$ESTs are used to locate and map genes that are actively expressed in a cell or tissue.$\newline$Based on this understanding, ESTs refer to all genes that are expressed as RNA.$\newline$Therefore, the correct option is Option 1: All genes that are expressed as RNA.