Left: spectroscopic overdensity of Cl J0227-0421. Plotted in black is the histogram of the number of galaxies with secure spectroscopic redshifts falling within a filter of the dimensions listed (χ refers to proper distance) for 1000 observations of random locations and central redshifts (2.5 <z_spec< 3.5) across the CFHTLS-D1 field avoiding gaps in spectroscopic coverage. The solid green line shows the best-fit Poisson distribution with the numbers to the left denoting the best-fit parameters. The number of members of Cl J0227-0421 with secure spectroscopic redshifts within the filter bounds is plotted as a red vertical dashed line. The Cl J0227-0421 galaxy overdensity, δ_gal ≡ (N_{gal,Proto − Struct.} − μ) /μ, is shown to the left of the vertical dashed line along with the formal significance of the spectroscopic overdensity, σ_{Proto − Struct.} ≡ (N_{gal,Proto − Struct.} − μ) /σ. Right: photometric redshift galaxy overdensity of Cl J0227-0421. The black histogram shows the Source Extractor (SEx) significance distribution of spurious density peaks in the CFHTLS-D1 field (see Sect. 3). The solid red line shows the best-fit Gaussian distribution to the significance distribution of the spurious peaks and the associated best-fit parameters are shown above this line. The horizontal dashed blue line denotes the SEx significance of the photometric redshift galaxy overdensity in Cl J0227-0421, while the number directly to the left of the line gives the formal significance of the overdensity that accounts for spurious density peaks.