Study will include 3 parts. Aim of Part 1 of this study is to establish the maximum tolerated
dose (MTD) and recommended dose for expansion (RDE) for M4344 (is an Ataxia Telangiectasia
Mutated and Rad3-related [ATR] inhibitors) in combination with niraparib in participants with
advanced solid tumors. Aim of Parts 2 and 3 of the study is to provide clinical
proof-of-concept for the preclinically predicted synergistic efficacy of ATR and
poly(ADP-Ribose) polymerase (PARP) inhibitors (PARPi) in defined populations of participants
with advanced breast cancer (aBC) with DDR mutations with an unmet medical need.