Get an expression vector of neighboring genes.
    Attribute:
        neighbors (list): List of gene identifiers of neighboring genes.
        gene_expression_dict (dict): (Gene identifier)-(gene expression) dictionary.

def get_neighbor_expression_vector(neighbors, gene_expression_dict):
    """Get an expression vector of neighboring genes.
    Attribute:
        neighbors (list): List of gene identifiers of neighboring genes.
        gene_expression_dict (dict): (Gene identifier)-(gene expression) dictionary.
    """
    expressions = []  # Expression vector.
    for gene in neighbors:
        try:
            expression = gene_expression_dict[gene]
        except KeyError:
            continue

        expressions.append(expression)
    return expressions