def preprocess_histograms_2(hists_orig, two_tail_fraction=0.05):
    """
    hists_orig: pandas.core.frame.DataFrame
    contains all MNI region histograms 
    for a single cbv volume .
    Returns: pandas.core.frame.DataFrame
    Preprocessing:
    
    1. Remove rows with only 0s .
    
    """
    hists = hists_orig.copy()
    
    # 1.
    hists = hists.loc[~(hists==0).all(axis=1)]
    return hists