Ube1yis a Y-linked gene transcribed in the testis, which maps to a region of the mouse Y required for normal spermatogonial proliferation.Ube1y,together with a ubiquitously expressed homologue on the X chromosome (Ube1x), encodes ubiquitin-activating enzyme E1, an enzyme essential for eukaryotic cell proliferation.Ube1yis thus a strong candidate for the Y function in spermatogonial proliferation. Using probes specific for the two genes, we have used Northern analysis and RNase protection to assess transcript levels throughout testis development and, by using germ cell-deficient XXSxr a testes and purified cell fractions, we have defined the testicular cell types in which transcription occurs.Ube1ytranscripts are already detectable in the fetal testis at 12.5 dpc, with higher levels at 14.5 dpc and then falling to low levels by the time of birth. Postnatally levels rise sharply, peaking at 10 dpp. Analysis of XXSxr a testes indicates that the bulk of theUbe1ytranscription is in germ cells. The analysis of purified cell fractions shows that X- and Y-encoded transcripts are present in A spermatogonia, both are at very low levels (or perhaps absent) in pachytene spermatocytes and then return to high levels in round spermatids. The reactivation of transcription in round spermatids implies a requirement for the ubiquitination pathway at this time. The presence ofUbe1xtranscripts in A spermatogonia raises the question as to whyUbe1ytranscripts are required. This question is discussed in relation to the spermatogenic failure in XSxr b O mice which are deleted forUbe1yand it is argued thatUbe1yserves to increase UBE1 production at a time of high demand.Ube1ytranscripts were also detected in XXY and XY ovaries.