  Recent ARPES measurement observed a large $ab$-axis gap anisotropy,
$\Delta(0,\pi)/\Delta(\pi,0)=1.5$, in clean YBa$_2$Cu$_3$O$_{7-\delta}$. This
indicates that some sub-dominant component may exist in the $d_{x^2-y^2}$-wave
dominant gap. We propose that the interlayer pairing tunneling contribution can
be determined through the investigation of the order parameter anisotropy.
Their potentially observable features in transport and spin dynamics are also
studied.
