Defining the units in which the eucaryotic genome is transcribed and translated is central to any analysis of eucaryotic gene expression. The relationship between heterogeneous nuclear RNA and messenger RNA raises the question of whether the primary transcript may be more complex than the sequence which is translated; as I concluded last month in the first part of this review, kinetic analyses of these two RNA populations provide some suggestive indications but cannot prove whether the nuclear population includes messenger precursors that are much longer than mature cytoplasmic messengers (Lewin, 1975). Here I discuss recent analyses of the sequence components present in hnRNA and mRNA and how they may be related to each other and to the organization of the genome.