HERV-H expression levels correlate with those of pluripotency transcription factors NANOG and OCT4 as human ES cells move down a differentiation pathway. N0, undifferentiated ES cells. N1, early initiation stage of differentiation. N2, neural progenitor stage. OCT4 and NANOG are positively correlated with HERV-H (ρ = 0.95, ρ = 0.84, respectively). SOX2 shows no correlation.