<｜begin▁of▁sentence｜>### Instruction:
Imagine you're working in a cutting-edge nanotechnology lab where you've conceptualized a molecular assembly line, expressed as a sequence of reactions starting from molecule 'A' and culminating in the desired product 'D'. The reaction pathway is represented by a series of intermediate steps, with 'A' transforming into 'B', 'B' into 'C', and finally 'C' into 'D'. To model and analyze the efficiency of this nanoscale process, you create a graph utilizing the reaction sequence data: [('A', 'B'), ('B', 'C'), ('C', 'D')].

Your task is to utilize NetworkX, a tool prevalent in computational research, to verify the integrity of the proposed molecular pathway. Your goal is to determine if the transition from the initial reactant 'A' to the final product 'D' is theoretically viable by ensuring a connected path exists in your graph model. This check will validate whether the molecules 'A', 'B', 'C', and 'D' indeed form a linear and uninterrupted reaction sequence as intended in your nanotechnological design.
### Response:
import networkx as nx

# Create a graph
G = nx.Graph()
G.add_edges_from([('A', 'B'), ('B', 'C'), ('C', 'D')])

# Define a path
path = ['A', 'B', 'C', 'D']

# Check if the path exists in the graph
path_exists = nx.is_path(G, path)
print('Path exists:' if path_exists else 'Path does not exist:')
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