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We present a comprehensive examination of the X-ray variability of the narrow line Seyfert 1 (NLS1) galaxy NGC 4051. We combine over 6.5 years of frequent monitoring observations by RXTE with a >100 ks continuous observation by XMM-Newton and so present a powerspectral density (PSD) covering an unprecedent frequency range of over 6.5 decades from <10-8 to >10-2 Hz. The combined RXTE and XMM-Newton PSD is a very good match to the PSD of the galactic black hole binary system (GBH) Cyg X-1 when in a ‘high’, rather than ‘low’, state providing the first definite confirmation of an AGN in a ‘high’ state.We find a break in the PSD at a frequency νB=8+4-3×10-4 Hz. If νB scales linearly with mass then, assuming a black hole (BH) mass of 10 M◎ for Cyg X-1, we imply a BH mass of 3+2-1×105 M◎ in NGC 4051, which is consistent with the recently reported reverberation value of 5+6-3×105 M◎. Hence NGC 4051 is emitting at ∼ 30% LEdd.We note that the higher energy photons lag the lower energy ones and that the lag is greater for variations of longer Fourier period and increases with the energy separation of the bands. Variations in different wavebands are very coherent at long Fourier periods but the coherence decreases at shorter periods and as the energy separation between bands increases. This behaviour is similar to that of GBHs and suggests a radial distribution of frequencies and photon energies with higher energies and higher frequencies being associated with smaller radii.It is not possible to fit all AGN to the same linear scaling of break timescale with black hole mass. Broad line AGN are consistent with a linear scaling of break timescale with mass from Cyg X-1 in its low state but NLS1 galaxies scale better with Cyg X-1 in its high state. We suggest that the relationship between black hole mass and break timescale is a function of another underlying parameter which may be accretion rate. [Copyright &y& Elsevier]